When stripping options, clear options if no fields remain and also cl…

…ear source code info (#251)

This updates the recently added
`options.StripSourceRetentionOptionsFromFile` so it is more thorough:
1. We were previously failing to strip source code locations for the
removed options. So now those are removed, too.
2. If we end up removing _all_ fields from an options message, set the
descriptor's options field to nil (effectively clearing it entirely,
instead of leaving it an empty message).

This now matches the output of `protoc` when it strips options.