dm cache metadata: remove unused struct 'thunk'

'thunk' has been unused since commit f177940a ("dm cache metadata:
switch to using the new cursor api for loading metadata").

Remove it.
Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

drivers/md/dm-cache-metadata.c

@@ -1282,15 +1282,6 @@ int dm_cache_insert_mapping(struct dm_cache_metadata *cmd,
 	return r;
 }
 
-struct thunk {
-	load_mapping_fn fn;
-	void *context;
-
-	struct dm_cache_metadata *cmd;
-	bool respect_dirty_flags;
-	bool hints_valid;
-};
-
 static bool policy_unchanged(struct dm_cache_metadata *cmd,
 			     struct dm_cache_policy *policy)
 {