refs.[ch]: remove unused ref_storage_backend_exists()

This function was added in 3dce444f17 (refs: add a backend method
structure, 2016-09-04), but has never been used by anything. The only
caller that might care uses find_ref_storage_backend() directly.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -33,11 +33,6 @@ static struct ref_storage_be *find_ref_storage_backend(const char *name)
 	return NULL;
 }
 
-int ref_storage_backend_exists(const char *name)
-{
-	return find_ref_storage_backend(name) != NULL;
-}
-
 /*
  * How to handle various characters in refnames:
  * 0: An acceptable character for refs

refs.h

@@ -836,8 +836,6 @@ int reflog_expire(const char *refname, const struct object_id *oid,
 		  reflog_expiry_cleanup_fn cleanup_fn,
 		  void *policy_cb_data);
 
-int ref_storage_backend_exists(const char *name);
-
 struct ref_store *get_main_ref_store(struct repository *r);
 
 /**