Remove expensive-to-clone fields from worktree's LocalSnapshot (#2497)

This fixes performance problems that @nathansobo and I have seen in some
cases, when a large number of files changed on disk. A lot of time was
being spent in `worktree::LocalSnapshot::clone`. I think this may have
been because of needing to clone the `removed_entry_ids` map. This
structure is only really used when *mutating* the `LocalSnapshot` in the
background scanner, so I moved it off of the snapshots.