`get_outputs()` for external subsystems is potentially unnecessary

[johnjasa]

Desired capability or behavior.

The get_outputs() function for SubsystemBuilderBase was a little confusing/misleading just based on the name and docstring (not really covered in the docs). It may not be necessary for some (all?) subsystems.

Solution is to check where and how this is used and refactor it away if possible.

Is your feature request related to a problem? Please describe.

No response

Associated Bug Report

No response