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