Revert the previous "signed commit" attempt

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

commit.h

@@ -175,9 +175,6 @@ struct commit_list *reduce_heads(struct commit_list *heads);
 
 extern int commit_tree(const char *msg, unsigned char *tree,
 		struct commit_list *parents, unsigned char *ret,
-		       const char *author, const char *sign_commit);
-
-extern int parse_signed_commit(const unsigned char *sha1,
-			       struct strbuf *message, struct strbuf *signature);
+		const char *author);
 
 #endif /* COMMIT_H */