Makefile: always remove .depend directories on 'make clean'

Makefile: always remove .depend directories on 'make clean'

Even if COMPUTE_HEADER_DEPENDENCIES is not set, some .o.d files
might be lying around from previous builds when it was.  This
is especially likely because using the CHECK_HEADER_DEPENDENCIES
feature requires building sometimes with COMPUTE... on and
sometimes with it off.  At the end of such an exercise, to get
a blank slate, the user ought to be able to just run 'make clean'.
Make it so.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>