Unify helpers IsRtpPacket and IsRtcpPacket

Bug: None
Change-Id: Ibe942de433435d256cd6827440136936d4b274d6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/225022
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34419}

test/rtp_header_parser.h

@@ -19,8 +19,6 @@ namespace webrtc {
 
 class RtpHeaderParser {
  public:
-  // Returns true if the packet is an RTCP packet, false otherwise.
-  static bool IsRtcp(const uint8_t* packet, size_t length);
   static absl::optional<uint32_t> GetSsrc(const uint8_t* packet, size_t length);
 };
 }  // namespace webrtc