Remove the now vestigal pyflakes tox env

The previous patch removes anything that runs the pyflakes env. This
patch removes the env itself becaus it's now not used.

Change-Id: I89d1f6669fc0b95d2944318504e15bd2101e8bce
Garbage collect Git repos in gerrit-git-prep

* modules/jenkins/files/slave_scripts/gerrit-git-prep.sh: Sometimes
Git repositories can be left in a dirty state, preventing subsequent
operations. If a git remote update fails, garbage collect and then
try again.

Change-Id: I8455a3193081f9a0c9372a10f5ffdbc25fc864d9
Fixes-Bug:1229352
Merge "Remove the now vestigal pyflakes tox env"
Merge "Garbage collect Git repos in gerrit-git-prep"