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