Please clean up qtbase repo in Gerrit

There's a lot of references in this repository, which makes Git push negotiation take a long time. It's noticeable when you're on a very slow network connection (like VPN over in-flight WiFi – I couldn't push at all).

$ git ls-remote gerrit | wc -l
532023

The biggest block that can probably be cleaned up are the refs/builds for the CI.

Try to see what gets negotiated, set in your environment GIT_TRACE_PACKET=1 while doing a git push.