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>

scripts/package/kernel.spec

@@ -107,11 +107,6 @@ elif [ -x /usr/bin/kernel-install ]; then
 kernel-install remove %{KERNELRELEASE}
 fi
 
-%postun
-if [ -x /sbin/update-bootloader ]; then
-/sbin/update-bootloader --remove %{KERNELRELEASE}
-fi
-
 %files -f %{buildroot}/kernel.list
 %defattr (-, root, root)
 %exclude /kernel.list