The docstring for _remove_from_gitolite_cache describes purpose of this PR, I'll just paste it here for convenience:

Gitolite has no notion of "deleting" a project and it can only add values to gl-conf.cache. Therefore we must manually wipe all entries related to a project when deleting it. If this method is not executed and if someone creates a project with the same fullname again then its gl-conf file won't get created (see link to commit below) and any subsequent invocation of gitolite trigger POST_COMPILE will fail, thus preventing creation of new repos/forks at the whole pagure instance.

See https://github.com/sitaramc/gitolite/commit/41b7885b77cfe992ad3c96d0b021ece51ce1b3e3 (later reverted upstream, but still used in most Pagure deployments).