i965/fs: Drop the viewport index/rtai clearing in gen6 fb writes.

These fields are documented to be in the payload, and though the FB
write docs say they *aren't* in the payload, for all other fields the
payload and header is structured so that no overwriting is required
except for non-default options.

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -2330,12 +2330,6 @@ fs_visitor::generate_fb_write(fs_inst *inst)
 		    brw_imm_ud(inst->target));
 	 }
 
-	 /* Clear viewport index, render target array index. */
-	 brw_AND(p, retype(brw_vec1_reg(BRW_MESSAGE_REGISTER_FILE, 0, 0),
-			   BRW_REGISTER_TYPE_UD),
-		 retype(brw_vec1_grf(0, 0), BRW_REGISTER_TYPE_UD),
-		 brw_imm_ud(0xf7ff));
-
 	 implied_header = brw_null_reg();
       } else {
 	 implied_header = retype(brw_vec8_grf(0, 0), BRW_REGISTER_TYPE_UW);