xwayland/selection: allow simultaneous Wayland-to-X11 transfers
xwayland/selection: allow simultaneous Wayland-to-X11 transfers

There seems to be no reason why we can't service multiple Wayland-to-X11
transfers concurrently, so long as they are to different windows (or
possibly, same windows but different target properties?)

This commit removes the queuing logic, but retains the request
de-duplication from #2428.

swaywm/wlroots,swaywm/wlroots,SirCmpwn/wlroots