dcsctp: remove unused WritePacketHeader method

BUG=None

Change-Id: Ieeb19ef976fe88a66a4a7b985f0600bb01044753
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/226945
Reviewed-by: Victor Boivie <boivie@webrtc.org>
Commit-Queue: Philipp Hancke <phancke@nvidia.com>
Cr-Commit-Position: refs/heads/master@{#34712}

net/dcsctp/packet/sctp_packet.h

@@ -77,7 +77,6 @@ class SctpPacket {
     std::vector<uint8_t> Build();
 
    private:
-    void WritePacketHeader();
     VerificationTag verification_tag_;
     uint16_t source_port_;
     uint16_t dest_port_;