screen-share: Don't unset server environment variables

screen-share: Don't unset server environment variables

There is no need to unset WAYLAND_DISPLAY and WAYLAND_SOCKET when screen-share
launches the fullscreen shell server. This was done originally in case the
launched server decided to use the wayland backend based on the presence of
these. However, we pass a command line argument telling it to use the RDP
backend, which overrides the automatic backend selection based on the
environment.

Keeping these environment variables allows the launched fullscreen shell server
to know the original server's display name, which it may need in order to show
a configuration UI.

Signed-off-by: Andrew Wedgbury <andrew.wedgbury@realvnc.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>