Remove unused CONTROL_FLAG_UNIDIRECTIONAL.

This CL lands server change 185170750 by bnc.

BUG=488484

Change-Id: I9d6b44ef24fe690fe707087bf2bcc366631a7933
Reviewed-on: https://chromium-review.googlesource.com/925806
Commit-Queue: Bence Béky <bnc@chromium.org>
Reviewed-by: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537776}

net/spdy/core/spdy_protocol.h

@@ -118,7 +118,6 @@ enum SpdyDataFlags {
 enum SpdyControlFlags {
   CONTROL_FLAG_NONE = 0x00,
   CONTROL_FLAG_FIN = 0x01,
-  CONTROL_FLAG_UNIDIRECTIONAL = 0x02,
 };
 
 enum SpdyPingFlags {