Remove the EC_SMALLMUL experimental flag.

This experiment has been fully adopted and is now an integral part
of the draft AV1 bitstream definition.

objdump -d libaom.a gives identical output before and after this
patch.

Change-Id: I6f936f4b10de23a9471e0ccadf9cf178fb62be69
diff --git a/aom_dsp/entdec.h b/aom_dsp/entdec.h
index e1145e8..35ac7fe 100644
--- a/aom_dsp/entdec.h
+++ b/aom_dsp/entdec.h
@@ -47,10 +47,8 @@
   const unsigned char *end;
   /*The read pointer for the entropy-coded bits.*/
   const unsigned char *bptr;
-  /*The difference between the coded value and the low end of the current
-     range.
-    {EC_SMALLMUL} The difference between the high end of the current range,
-     (low + rng), and the coded value, minus 1.
+  /*The difference between the high end of the current range, (low + rng), and
+     the coded value, minus 1.
     This stores up to OD_EC_WINDOW_SIZE bits of that difference, but the
      decoder only uses the top 16 bits of the window to decode the next symbol.
     As we shift up during renormalization, if we don't have enough bits left in