Remove unused enum RTPDirections.

BUG=

Review URL: https://codereview.webrtc.org/1582523002

Cr-Commit-Position: refs/heads/master@{#11221}
diff --git a/webrtc/common_types.h b/webrtc/common_types.h
index 048485f..6d1886b 100644
--- a/webrtc/common_types.h
+++ b/webrtc/common_types.h
@@ -311,12 +311,6 @@
 // RTP
 enum {kRtpCsrcSize = 15}; // RFC 3550 page 13
 
-enum RTPDirections
-{
-    kRtpIncoming = 0,
-    kRtpOutgoing
-};
-
 enum PayloadFrequencies
 {
     kFreq8000Hz = 8000,