kbuild: rpm-pkg: remove unneeded KERNELRELEASE from modules/headers_install

This is a temporary workaround added by commit f6e09b07 ("kbuild:
do not put .scmversion into the source tarball").

Since commit 1cb86b6c ("kbuild: save overridden KERNELRELEASE in
include/config/kernel.release"), the user-supplied KERNELRELEASE is
saved in include/config/kernel.release.

Remove it again.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>