Revert "kbuild/mkspec: support 'update-bootloader'-based systems"

This reverts commit 27c3bffd.

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

However, I'd like to understand why 'update-bootloader --remove' is
needed for uninstallation, while 'update-bootloader --add' was not
called during the installation.

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