Remove workaround for old mkinitramfs-kpkg

* Makefile (install-client-nokey): Do not install
  "initramfs-tools-hook-conf".
* debian/control (mandos-client/Depends): Add "(>= 0.99)" to
  dependency on "initramfs-tools", since that is the version which
  removed mkinitramfs-kpkg.
* debian/mandos-client.dirs: Remove
  "usr/share/initramfs-tools/conf-hooks.d".
* initramfs-tools-hook-conf: Removed.