libweston-desktop: rename weston_desktop_client_destroy()

This function here is a wl_resource destructor, but we will need another
function for externally triggered destroy when wl_resource does not
exist.

Rename the existing function, because the old name fits better the new
function to be written.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>