kbuild,gcov: remove unnecessary workaround

Since commit 371fdc77 (kbuild: collect shorthands into
scripts/Kbuild.include), scripts/Makefile.clean includes
scripts/Kbuild.include.

The workaround and the comment block in kernel/gcov/Makefile
are no longer necessary.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>