[3.x] Do not set gravity and damping when Custom Integrator is enabled.

[madmiraal]

3.x version of #42650.

Fixes #40508.

[rburing]

The code itself looks good. Could you just explain why putting it here guarantees that it is always run? (I'm not that familiar with the Bullet physics server.)

[madmiraal]

As specified in the docs when custom_integrator is enabled:

internal force integration will be disabled (like gravity or air friction) for this body.

This is where internal gravity and air friction force integration is calculated, so this PR simply checks whether custom_integrator (Godot renames this variable internally to omit_force_integration) is enabled, and, if so, sets the internal force to zero (in case it's not) and returns.

[akien-mga]

Thanks!