tests/test-gendoc-it.t

rebase: delete seemingly unnecessary needupdate()

This seemed to be about checking that the user hasn't updated away
when we asked them to resolve merge conflicts. These days we call
`cmdutil.checkunfinished()` and refuse to update, so the user
shouldn't be able to get into this state.

`test-rebase-interruptions.t` actually has some tests where it
disables the rebase extension in order to be allowed to do some of
these updates. That still passes, but I wouldn't personally haved
cared if that failed.

Differential Revision: https://phab.mercurial-scm.org/D7825

#require docutils gettext

  $ $TESTDIR/check-gendoc it
  checking for parse errors