Revert "google+ bug fix for overriding CallbackHandler.finish()"

oops, nevermind. the real problem is that overriding finish() isn't supported because google-api-python-client wants to own the entire callback handler. bleh. added to the TODO list.

This reverts commit feec3e1fbc3b93d66e193bbb12db81eb80bb6e26.
Merge branch 'master' of github.com:snarfed/oauth-dropins