st/mesa: Remove NULL check of pointer that could not be NULL.

cbuf has been dereferenced twice earlier in the same if-block. It is
either not NULL or a crash has already occurred.

src/mesa/state_tracker/st_atom_constbuf.c

@@ -84,7 +84,6 @@ void st_upload_constants( struct st_context *st,
       }
 
       /* load Mesa constants into the constant buffer */
-      if (cbuf)
       st_no_flush_pipe_buffer_write(st, *cbuf,
 				    0, paramBytes,
 				    params->ParameterValues);