backend-vnc: Remove the display when output is disabled

This prevents a potential abort yelling:

PANIC: ../subprojects/neatvnc/src/server.c: 2245: Multiple displays are not implemented. Aborting!

which happens because we never unregister the output, a mirror function
for nvnc_add_display().

Further more, we migrate nvc_close() to vnc_destroy() and check for
a valid output in vnc_client_cleanup(). Having a single caller in
vnc_shutdown() (which was migrated) we removed that entirely. Necessary
to avoid a use-after-free caused by nvc_close().

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