Removes duplicated cassie configuration in core extension (#383)

# Description

The `cassie.py` under the `omni.isaac.orbit` extension looks duplicated
with the one under `omni.isaac.orbit_assets`, and after a search in the
folder, I found that it's not being used or referenced at all. This MR
removes it to avoid potential confusion.

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have run all the tests with `./orbit.sh --test` and they pass
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

CONTRIBUTORS.md

@@ -38,6 +38,7 @@ Guidelines for modifications:
 * Chenyu Yang
 * Jia Lin Yuan
 * Jingzhou Liu
+* Muhong Guo
 * Kourosh Darvish
 * Özhan Özen
 * Qinxi Yu

source/extensions/omni.isaac.orbit/omni/isaac/orbit/assets/config/cassie.py

-# Copyright (c) 2022-2024, The ORBIT Project Developers.
-# All rights reserved.
-#
-# SPDX-License-Identifier: BSD-3-Clause
-
-"""Configuration for Agility robots.
-
-The following configurations are available:
-
-* :obj:`CASSIE_CFG`: Agility Cassie robot with simple PD controller for the legs
-
-Reference: https://github.com/UMich-BipedLab/Cassie_Model/blob/master/urdf/cassie.urdf
-"""
-
-
-import omni.isaac.orbit.sim as sim_utils
-from omni.isaac.orbit.actuators import ImplicitActuatorCfg
-from omni.isaac.orbit.utils.assets import ISAAC_ORBIT_NUCLEUS_DIR
-
-from ..articulation import ArticulationCfg
-
-##
-# Configuration
-##
-
-
-CASSIE_CFG = ArticulationCfg(
-    spawn=sim_utils.UsdFileCfg(
-        usd_path=f"{ISAAC_ORBIT_NUCLEUS_DIR}/Robots/Agility/Cassie/cassie.usd",
-        activate_contact_sensors=True,
-        rigid_props=sim_utils.RigidBodyPropertiesCfg(
-            disable_gravity=False,
-            retain_accelerations=False,
-            linear_damping=0.0,
-            angular_damping=0.0,
-            max_linear_velocity=1000.0,
-            max_angular_velocity=1000.0,
-            max_depenetration_velocity=1.0,
-        ),
-        articulation_props=sim_utils.ArticulationRootPropertiesCfg(
-            enabled_self_collisions=True, solver_position_iteration_count=4, solver_velocity_iteration_count=0
-        ),
-    ),
-    init_state=ArticulationCfg.InitialStateCfg(
-        pos=(0.0, 0.0, 0.9),
-        joint_pos={
-            "hip_abduction_left": 0.1,
-            "hip_rotation_left": 0.0,
-            "hip_flexion_left": 1.0,
-            "thigh_joint_left": -1.8,
-            "ankle_joint_left": 1.57,
-            "toe_joint_left": -1.57,
-            "hip_abduction_right": -0.1,
-            "hip_rotation_right": 0.0,
-            "hip_flexion_right": 1.0,
-            "thigh_joint_right": -1.8,
-            "ankle_joint_right": 1.57,
-            "toe_joint_right": -1.57,
-        },
-        joint_vel={".*": 0.0},
-    ),
-    soft_joint_pos_limit_factor=0.9,
-    actuators={
-        "legs": ImplicitActuatorCfg(
-            joint_names_expr=["hip_.*", "thigh_.*", "ankle_.*"],
-            effort_limit=200.0,
-            velocity_limit=10.0,
-            stiffness={
-                "hip_abduction.*": 100.0,
-                "hip_rotation.*": 100.0,
-                "hip_flexion.*": 200.0,
-                "thigh_joint.*": 200.0,
-                "ankle_joint.*": 200.0,
-            },
-            damping={
-                "hip_abduction.*": 3.0,
-                "hip_rotation.*": 3.0,
-                "hip_flexion.*": 6.0,
-                "thigh_joint.*": 6.0,
-                "ankle_joint.*": 6.0,
-            },
-        ),
-        "toes": ImplicitActuatorCfg(
-            joint_names_expr=["toe_.*"],
-            effort_limit=20.0,
-            velocity_limit=10.0,
-            stiffness={
-                "toe_joint.*": 20.0,
-            },
-            damping={
-                "toe_joint.*": 1.0,
-            },
-        ),
-    },
-)