mesa/st: remove redundant call to st_finish in CopyTexSubImage

Rendering should already have been flushed, any synchronization will
be done by the driver or memory manager.

src/mesa/state_tracker/st_cb_texture.c

@@ -1315,9 +1315,6 @@ st_copy_texsubimage(GLcontext *ctx,
    GLboolean use_fallback = GL_TRUE;
    GLboolean matching_base_formats;
 
-   /* any rendering in progress must complete before we grab the fb image */
-   st_finish(ctx->st);
-
    /* make sure finalize_textures has been called? 
     */
    if (0) st_validate_state(ctx->st);