Revert "selftests: Fix O= and KBUILD_OUTPUT handling for relative paths"

This reverts commit 303e6218.

This patch breaks several CI use-cases that run kselftest builds
without using main Makefile. This fix depends on abs_objtree which
is undefined when kselftest build is invoked on selftests Makefile
without going through the main Makefile.

Revert this for now as this patch impacts selftest runs.

Fixes: 303e6218

 ("selftests: Fix O= and KBUILD_OUTPUT handling for relative paths")
Reported-by: Cristian Marussi <cristian.marussi@arm.com>
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>