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().
Signed-off-by: Marius Vlad marius.vlad@collabora.com
Observed only when we have handler for compositor signals (should be introduced with !1476 (merged))
/cc @pH5