magit-find-file-files: refactor

* Instead of taking an argument, always bind `default-directory'
  to value of `magit-get-top-dir'.
* Use `--remove' instead of `cl-remote-if'.
magit-find-file-completing-read: bind default-directory directly

Don't worry about the case when it is nil, since we do nothing but raise
an error in that case anyway.  Use `-if-let'.  And some other cosmetics.