Only set GMX_PYTHON_PACKAGE=ON where needed.

Remove GMX_PYTHON_PACKAGE=ON from default CMAKE_GMXAPI_OPTIONS. Only provide it in the jobs supporting documentation builds (which use a gmxapi Python package staged in the build tree).

This change will better isolate the purposes of the jobs and make it easier to get useful information from their build results.

Also, remove a "TODO" that is confusing and presumably outdated.