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
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate