refactor: update MuJoCo XML and STL files, remove outdated ones

[lil-km]

Changes Made

This PR updates the MuJoCo XML description and STL model files to enhance simulation accuracy and visual representation. It also removes outdated files.

Added/Modified:

• Updated XML files for improved robot simulation accuracy
• Updated environment scenes XML files to be more structured, now including the same robot arm file consistently
• Revised STL files for better visual representation
• Modified environment to account for the change in the end-effector name ("link_6")

Removed:

• Unused STL model files

[perezjln]

Thanks @lil-km I believe this refact. of the robot definition makes sense and hopefully will help producing more environment maybe more easily and consistently.