waylandsink: remove the ugly gst_wl_display_stop() now that this mechanism is not...

waylandsink: remove the ugly gst_wl_display_stop() now that this mechanism is not needed anymore

Because we no longer have a custom buffer pool that holds a reference
to the display, there is no way for a cyclic reference to happen like
before, so we no longer need to explicitly call a function from the
display to release the wl_buffers.

However, the general mechanism of registering buffers to the display
and forcibly releasing them when the display is destroyed is still
needed to avoid potential memory leaks. The comment in wlbuffer.c
is updated to reflect the current situation.