kbuild: pacman-pkg: do not override objtree

kbuild: pacman-pkg: do not override objtree

objtree is defined and exported by the top-level Makefile. I prefer
not to override it.

There is no need to pass the absolute path of objtree. PKGBUILD can
detect it by itself.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by:  Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Christian Heusel <christian@heusel.eu>