kbuild: buildtar: Remove unused $dirs

The shell variable $dirs is not used any more since 1fc90958
("kbuild: tar-pkg: use tar rules in scripts/Makefile.package"),
therefore remove it".

Fixes: 1fc90958

 ("kbuild: tar-pkg: use tar rules in scripts/Makefile.package")
Signed-off-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>