rebase: stop clearing on-disk mergestate when running in memory
In-memory merge no longer uses on-disk mergestate since
19590b126764
(merge: use in-memory mergestate when using in-memory context,
2020-09-15). This patch removes the clearing from two places in the
in-memory rebase code. I've verified that the one with a TODO was
indeed fixed by the aforementioned commit. The other instance doesn't
seem to have any test coverage, but I'm pretty confident it's also
safe to remove.
Differential Revision: https://phab.mercurial-scm.org/D9057
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors