pushrebase: remove check-bookmarks parts generator
 adds a new check which moves bookmarks race check earlier, and this conflicts
with pushrebase. In reality pushrebase-enabled repos don't need this check
anyway, so we can just remove this generator.
Test Plan: - run pushrebase tests, see them change their status from failing to passing
Differential Revision: https://phab.mercurial-scm.org/D1655