[intel] Remove gratuitous (batchbuffer) flush before doing buffer clears.

Increases OA performance by about 3%.

src/mesa/drivers/dri/intel/intel_buffers.c

@@ -570,8 +570,6 @@ intelClear(GLcontext *ctx, GLbitfield mask)
       }
    }
 
-   intelFlush(ctx);             /* XXX intelClearWithBlit also does this */
-
    if (blit_mask) {
       if (INTEL_DEBUG & DEBUG_BLIT) {
 	 DBG("blit clear:");