Remove code for catching an assertion error in github-flask

The github-flask extension was raising an assertion error in rare cases.
This was fixed in version 1.0.0.

See changeset at
https://github.com/cenkalti/github-flask/commit/be792fccdfd02d564aae008ab459ddf266f01796

This commit updates to the newest version of the github-flask extension
and removes the code that was needed to deal with the assertion error.