hmi-controller: Add missing removal of destroy listener

Shutting down the compositor gives us:

Invalid write of size 8
   at 0x4B1AEDB: wl_list_remove (wayland-util.c:56)
   by 0x4AF05BF: weston_signal_emit_mutable (signal.c:66)
   by 0x4ACBC2C: weston_compositor_destroy (compositor.c:8629)
   by 0x4864A4B: wet_main (main.c:3908)
   by 0x10915D: main (executable.c:33)
 Address 0x17435f20 is 224 bytes inside a block of size 384 free'd
   at 0x484617B: free (vg_replace_malloc.c:872)
   by 0x17718C7E: hmi_controller_destroy (hmi-controller.c:761)
   by 0x4AF059A: weston_signal_emit_mutable (signal.c:62)
   by 0x4ACBC2C: weston_compositor_destroy (compositor.c:8629)
   by 0x4864A4B: wet_main (main.c:3908)
   by 0x10915D: main (executable.c:33)

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>