Fix joints and actuators

[squarefk]

Fix two minor issues:

1. Address the issue where some joints remain in after removing joints from the XML.
2. Ensure proper handling when joint_id and actuator_id have different orders, and utilize the gear attribute from the actuator.

[squarefk]

The relevant section in the xml file looks like this:

[kevin-thankyou-lin]

Thanks @squarefk! Double checking you've tested the changes with the GR1 too?

[squarefk]

@kevin-thankyou-lin Yes, I have tested it. If no attribute is explicitly declared in the robot's XML file, the value read from MjModel.actuator_gear will default to 1.

[kevin-thankyou-lin]

lgtm

[kevin-thankyou-lin]

thanks!