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.

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"

src/refdb_fs.h

-/*
- * Copyright (C) the libgit2 contributors. All rights reserved.
- *
- * This file is part of libgit2, distributed under the GNU GPL v2 with
- * a Linking Exception. For full terms see the included COPYING file.
- */
-#ifndef INCLUDE_refdb_fs_h__
-#define INCLUDE_refdb_fs_h__
-
-#include "common.h"
-
-#include "strmap.h"
-
-typedef struct {
-	git_strmap *packfile;
-	time_t packfile_time;
-} git_refcache;
-
-#endif