xwayland: remove unused parameter of xwl_glamor_pixmap_get_wl_buffer

There are currently no callers that make use of the "created" output parameter
of xwl_glamor_pixmap_get_wl_buffer. Remove it, along with the corresponding
argument of the associated EGL backend entrypoint.
4 jobs for remove-unused-param in 5 minutes and 39 seconds (queued for 1 second)