[antlir][reddiff] nuke image_rpmbuild

Summary:
The only non-internal-test usage of this was `lu/systemd/daily` which has now
been moved to just use hyperscale rpms

Test Plan: burn_it_down_sunday

Reviewed By: epilatow

Differential Revision: D51264340

fbshipit-source-id: b19e09a5d4f25127d576a76471ba5d2d75beda69