Support rectangular tx_size distortion calculation by default

Remove the CONFIG_EXT_TX flag.

Change-Id: I7e35036156e116a2c00c982a8d57473258e002ea
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index d85d3ea..d68e6ef 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -1121,7 +1121,6 @@
                                TX_SIZE tx_size) {
   uint64_t sse;
   switch (tx_size) {
-#if CONFIG_EXT_TX
     case TX_4X8:
       sse = aom_sum_squares_2d_i16(diff, diff_stride, 4) +
             aom_sum_squares_2d_i16(diff + 4 * diff_stride, diff_stride, 4);
@@ -1146,7 +1145,6 @@
       sse = aom_sum_squares_2d_i16(diff, diff_stride, 16) +
             aom_sum_squares_2d_i16(diff + 16, diff_stride, 16);
       break;
-#endif  // CONFIG_EXT_TX
     default:
       assert(tx_size < TX_SIZES);
       sse = aom_sum_squares_2d_i16(diff, diff_stride, tx_size_wide[tx_size]);