Remove Github Credential Authentication now that it's expired in November 2020

[particthistle]

Steps to reproduce the issue

While assisting some students with GSoC tonight, Philip Walton and I spotted the alert when Credentials are selected as the GitHub Authentication method.

Should the credentials method be removed in the next version of Patch Tester now that it's been deprecated by GitHub?

Documentation on the Patch Tester that I'm aware of so far already refers to using the Token method, so taking it out should not impact users.
https://docs.joomla.org/Testing_Joomla!_patches
https://magazine.joomla.org/all-issues/june-2020/github-installing-git

[particthistle]

Closing as #320 addresses the issue.