refactor(client): Remove oauth_sign_in and oauth_sign_up, their functionality is completely handled by sign_in/sign_up and the brokers.

fixes #1841
Merge pull request #1932 from mozilla/issue-1841-remove-oauth-views

refactor(client): Remove oauth_sign_in and oauth_sign_up, their functionality is completely handled by sign_in/sign_up and the brokers.