virgl: remove unused variable

This variable is now unused, so let's remove it.

Fixes: db77573d7b (virgl: modify how we handle GL_MAP_FLUSH_EXPLICIT_BIT)
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>

src/gallium/drivers/virgl/virgl_context.c

@@ -784,7 +784,6 @@ static void virgl_flush_from_st(struct pipe_context *ctx,
                                enum pipe_flush_flags flags)
 {
    struct virgl_context *vctx = virgl_context(ctx);
-   struct virgl_screen *rs = virgl_screen(ctx->screen);
 
    if (flags & PIPE_FLUSH_FENCE_FD)
        vctx->cbuf->needs_out_fence_fd = true;