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 black
$ cd $RUNTESTDIR/..
$ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/** - "contrib/python-zstandard/**"'`