Remove gazebo plugin related code from USD -> SDF converter

[adlarkin]

Desired behavior

The USD -> SDF converter in the usd component of the SDFormat repository should not handle gazebo plugins, since SDFormat is meant to be a generic library that is not explicitly tied to the gazebo simulator (currently, the converter is handling gazebo plugins).

Alternatives considered

• Give users the option to opt-out of having gazebo plugins in the converted SDF file when running the USD -> SDF converter (this approach was taken in USD to SDF: Added diff drive plugin sdformat#904)

Implementation suggestion

It may be worth having a gazebo-specific converter/"enhancer" for USD -> SDF that handles things like gazebo plugins. This could be placed in the ign-gazebo repository. The downside to this would be that users have to go through two steps to get the SDF file they want if they're looking to include gazebo plugins (run the USD -> SDF converter in SDFormat, and then run this gazebo-specific tool in ign-gazebo). I imagine there are other alternatives that I cannot think of at the moment.

Additional context

The initial conversation that prompted the creation of this issue can be found here: gazebosim/sdformat#904 (comment)