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>