kbuild: remove "rpm" target, which is alias of "rpm-pkg"

As commit ebaad7d3 ("kbuild: rpm: prompt to use "rpm-pkg" if "rpm"
target is used") noticed, the "rpm" target is now removed.
I assume people have already migrated to "rpm-pkg".

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