Silience a compiler warning in build_inter_predictors()

Remove the unused variable when cb4x4 is enabled.

Change-Id: I0d4d9ebd989cf90b9373d9989dd5e78b93d59e25
diff --git a/av1/common/reconinter.c b/av1/common/reconinter.c
index bbab258..3d89715 100644
--- a/av1/common/reconinter.c
+++ b/av1/common/reconinter.c
@@ -900,7 +900,9 @@
   struct macroblockd_plane *const pd = &xd->plane[plane];
 #if CONFIG_MOTION_VAR
   const MODE_INFO *mi = xd->mi[mi_col_offset + xd->mi_stride * mi_row_offset];
+#if !CONFIG_CB4X4
   const int build_for_obmc = !(mi_col_offset == 0 && mi_row_offset == 0);
+#endif  // !CONFIG_CB4X4
 #else
   const MODE_INFO *mi = xd->mi[0];
 #endif  // CONFIG_MOTION_VAR