Use hex search for realtime mode speed>4

Test showed using hex search in realtime mode largely speed up
encoding process, and still achieves similar quality like the
diamond search we have. Therefore, removed the diamond search
option.

Change-Id: I975767d0ec0539f9f6ed7fdfc09506e39761b66c

vp8/encoder/onyx_if.c

@@ -980,11 +980,9 @@ void vp8_set_speed_features(VP8_COMP *cpi)
         {
             sf->auto_filter = 0;                     // Faster selection of loop filter
 
-#if CONFIG_REALTIME_ONLY
             sf->search_method = HEX;
-#else
-            sf->search_method = DIAMOND;
-#endif
+            //sf->search_method = DIAMOND;
+
             sf->iterative_sub_pixel = 0;
 
             cpi->mode_check_freq[THR_V_PRED] = 4;