kbuild: remove duplicated arch/*/include/generated/uapi include path

Commit 90ac086b ("Makefile: include arch/*/include/generated/uapi
before .../generated") introduced this for bisect'ability.  The commit
chose to promote arch/*/include/generated/uapi in the search path
rather than cleaning stale headers.

After all, we found that approach was not enough, and ended up with
cleaning stale headers by commit cda2c65f ("kbuild: Remove stale
asm-generic wrappers").

So, the extra search path is no longer needed because Kbuild invokes
scripts/Makefile.asm-generic and remove stale headers before it starts
descending.

This commit is also reverting commit dc33db7c ("Kbuild: avoid
duplicate include path") because we have no more duplicated path.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>