Revert "ANDROID: Kbuild: Support KBUILD_MIXED_TREE in Makefile.modfinal"

This reverts commit 6b793353.

It causes lots of merge conflicts with upstream changes that come into
the tree in 5.16-rc1, so revert it for now.  If this is really still
needed, it can come back later, ideally it should go upstream first.

Bug: 202075496
Cc: Yifan Hong <elsk@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I994ce12885e06bb984c1b8cbe3d9c5663f77bb51