refdb_fs: remove unused header file

The "refdb_fs.h" header contains a single struct `git_refcache` that is
not used anywhere. As a result, we can just delete the header altogether
as it doesn't have any purpose and may confuse readers.

Author: pks-t

src/refdb_fs.c

@@ -5,8 +5,6 @@
  * a Linking Exception. For full terms see the included COPYING file.
  */
 
-#include "refdb_fs.h"
-
 #include "refs.h"
 #include "hash.h"
 #include "repository.h"