Removing unused alt_freq field from VP9EncoderConfig.

Change-Id: I9b683c8647a864e74073161f4aa6f2911b7825e3

vp9/encoder/vp9_encoder.h

diff --git a/vp9/encoder/vp9_encoder.h b/vp9/encoder/vp9_encoder.h
index 1ba7634..c69a345 100644
--- a/vp9/encoder/vp9_encoder.h
+++ b/vp9/encoder/vp9_encoder.h
@@ -256,7 +256,6 @@
 
   // these parameters aren't to be used in final build don't use!!!
   int play_alternate;
-  int alt_freq;
 
   int encode_breakout;  // early breakout : for video conf recommend 800