Revert old fix for #617

This partially reverts commit ee1aff916af66bea3a8eae96adc83eb1a215029e.

Trying to handle the issue in pb_common.c is misguided.
The generator has now been fixed to keep the descriptor array in sorted order.
diff --git a/pb_common.c b/pb_common.c
index e764735..6aee76b 100644
--- a/pb_common.c
+++ b/pb_common.c
@@ -207,7 +207,7 @@
         pb_size_t start = iter->index;
         uint32_t fieldinfo;
 
-        if (tag < iter->tag && PB_HTYPE(iter->type) != PB_HTYPE_ONEOF)
+        if (tag < iter->tag)
         {
             /* Fields are in tag number order, so we know that tag is between
              * 0 and our start position. Setting index to end forces