pushrebase: remove check-bookmarks parts generator

Summary:
[1] 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.

[1] https://www.mercurial-scm.org/repo/hg/rev/ad5f2b923b0d

Test Plan: - run pushrebase tests, see them change their status from failing to passing

Reviewers: #fbhgext

Differential Revision: https://phab.mercurial-scm.org/D1655