Remove zz_installkernel and zz_removekernel links

Older versions of systemd did not provide post install and post remove
scripts to update the boot configuration. The latest version of systemd
does, so we no longer need to patch in our own symlinks to achieve this.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

debian/systemd-boot.links

-# Links for adding/removing kernel and initramfs
-/usr/bin/kernel-install /etc/kernel/postinst.d/zz_installkernel
-/usr/bin/kernel-install /etc/kernel/postrm.d/zz_removekernel
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