Since replacing Diffusion with Gitiles as Gerrit's repo viewer, we've greatly reduced the utility of having Git repositories in Phabricator. We should do the following:
* For all repos that have not yet been referenced in a comment or other config/etc, drop them
* Creating a repository in Phabricator should be as-needed instead of assumed for all Gerrit repos
* Archived repos can basically all be deleted, provided they satisfy the first condition
* Uninstall the Gerrit-To-Phab plugin we wrote for 1:1 mapping. It's not necessary anymore
Also means that T183792, T115624, probably a few others can be declined.