Makefile: remove a cleaning target "tidy"

Before this commit, "make tidy" did
"make clean" + delete "*.depend*" files.

But, we do not have "*.depend*" files any more,
which means "make tidy" is the same as "make clean".

This commit removes the redandant target "tidy".

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

MAKEALL

@@ -702,7 +702,7 @@ build_target() {
 	if [ $BUILD_MANY == 1 ] ; then
 		trap - TERM
 
-		${MAKE} -s tidy
+		${MAKE} -s clean
 
 		if [ -s ${LOG_DIR}/${target}.ERR ] ; then
 			cp ${LOG_DIR}/${target}.ERR ${OUTPUT_PREFIX}/ERR/${target}