mergestate: remove unnecessary clearing of `localctx` and `otherctx`

Authored by martinvonz.


mergestate: remove unnecessary clearing of localctx and otherctx

As noted in the previous commit, there are no callers that (re-)use
the instance after calling reset(). There are also no callers that
call _read() after doing anything with the instance (it's only
called right after an instance is created).

If reviewers feel that this is too risky, I can extract and reuse the
poisoning code that indygreg once added for poisining repo instances.

Differential Revision: https://phab.mercurial-scm.org/D9036