localrepo: drop `_cacheabletip` method It iss dead code now.

branchmap: drop `_cacheabletip` usage in `updatecache` Nobody overwrite the `_cacheabletip` any more. We always update the cache for the whole repo and write it to disk (or at list try to). The `updatecache` code is simplied to remove the double phase logic associated with _cacheabletip.

mq: drop `_cacheabletip` usage Strip have dedicated work around to solve the same problem, strip is even a fraction faster without that thanks to simpler update process of the branchcache.