Revert "kbuild/mkspec: clean boot loader configuration on rpm removal"

This reverts commit 6ef41e22.

If this is still needed, we can bring it back.

However, I'd like to understand why 'new-kernel-pkg --remove' is
needed for uninstallation, while 'new-kernel-pkg --install' was not
called during the installation.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>