fsck: remove the unused MISSING_TREE_OBJECT

This error type has never been used since it was introduced at
159e7b08 (fsck: detect gitmodules files, 2018-05-02).

Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsck.h

@@ -39,7 +39,6 @@ enum fsck_msg_type {
 	FUNC(MISSING_TAG, ERROR) \
 	FUNC(MISSING_TAG_ENTRY, ERROR) \
 	FUNC(MISSING_TREE, ERROR) \
-	FUNC(MISSING_TREE_OBJECT, ERROR) \
 	FUNC(MISSING_TYPE, ERROR) \
 	FUNC(MISSING_TYPE_ENTRY, ERROR) \
 	FUNC(MULTIPLE_AUTHORS, ERROR) \