[cleanup] return request.submit() directly
[cleanup] return request.submit() directly

- A lot of methods assign request.submit() to a data variable
  and return data. Now just return request.submit() directly
  as data is not used elsewhere.
- Remove non-useful comments
- return None as default in image_repository()

Change-Id: I3c5bef5b7b4c88f99ef1909a4c89032095267304

wikimedia/pywikibot-core,wikimedia/pywikibot-core