Skip calling `ResultCache.cleanup` when files size is 1
Skip calling `ResultCache.cleanup` when files size is 1

For performance 🚀

* `ResultCache.cleanup` takes time.
  * If there are 8000 cache files, it takes about 200ms.
* It is meaningful to save as much as 200ms when integrating text editors.
* When rubocop called by text editor, this `files.size` will be always 1.

We will lose the opportunity for cleaning when the cache file is increased by 1,
but I believe it does not seem important. :)

Co-Authored-By: Connor Shea <ce5845b12f2f1d773f833057124e88241e8fbaf1@gmail.com>

tdeo/rubocop,tdeo/rubocop,tejasbubane/rubocop,jmks/rubocop,bbatsov/rubocop,maxjacobson/rubocop,jmks/rubocop,tejasbubane/rubocop,meganemura/rubocop,deivid-rodriguez/rubocop,meganemura/rubocop,deivid-rodriguez/rubocop,smakagon/rubocop,panthomakos/rubocop,mikegee/rubocop,tdeo/rubocop,panthomakos/rubocop,rrosenblum/rubocop,jfelchner/rubocop,rrosenblum/rubocop,bbatsov/rubocop,jfelchner/rubocop,jmks/rubocop,mikegee/rubocop,smakagon/rubocop,mikegee/rubocop,maxjacobson/rubocop,panthomakos/rubocop,meganemura/rubocop,maxjacobson/rubocop,jfelchner/rubocop,tejasbubane/rubocop,deivid-rodriguez/rubocop,rrosenblum/rubocop