Cleanup IsSquashInProgress
With #3713 (closed), we now support full in-memory squashing and as a result we don't need to create worktrees anymore. This breaks IsSquashInProgress, which used to check for worktrees to see whether there's any pending squashes or not. Given that this function will now always return false, we should clean it up and remove all of its callers as soon as we have fully enabled in-memory squashing.