vc-diff: Delete redundant call to vc-maybe-buffer-sync

vc-diff: Delete redundant call to vc-maybe-buffer-sync

This call is redundant with the subsequent call to
vc-buffer-sync-fileset.  I believe this call to
vc-maybe-buffer-sync was reintroduced by accident -- the
changeset adding the call to vc-buffer-sync-fileset deleted a
call to vc-sync-fileset in the same place.

* lisp/vc/vc.el (vc-diff): Delete redundant call to
vc-maybe-buffer-sync.
* lisp/files.el (find-buffer-visiting): Note in docstring that
this function only ever returns base buffers.

(cherry picked from commit d047a89e769f3c8429c43a40d5f251a895590d3a)