kbuild: do not put .scmversion into the source tarball

.scmversion is used by (src)rpm-pkg and deb-pkg to carry KERNELRELEASE.

In fact, deb-pkg does not rely on it any more because the generated
debian/rules specifies KERNELRELEASE from the command line.

Do likwise for (src)rpm-pkg, and remove this feature.
For the same reason, you do not need to save LOCALVERSION in the
spec file.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>