Revert "Add transaction id to CandidatePairEvents."

This reverts commit c308bdfa451ee2ceac7096b6777fcbf756f4091a.

Reason for revert: The msan bot has been consistently failing since this commit. See eg https://ci.chromium.org/p/webrtc/builders/luci.webrtc.ci/Linux%20MSan/16989

Original change's description:
> Add transaction id to CandidatePairEvents.
> 
> The transaction id is a randomly generated number used to link stun
> requests and responses (https://tools.ietf.org/html/rfc5389#section-6).
> Logging this will help us debug ICE network issues.
> 
> Bug: webrtc:9972
> Change-Id: I93167cb119aad99156e8727b6e4eeeff5198f924
> Reviewed-on: https://webrtc-review.googlesource.com/c/109720
> Commit-Queue: Zach Stein <zstein@webrtc.org>
> Reviewed-by: Qingsi Wang <qingsi@webrtc.org>
> Reviewed-by: Elad Alon <eladalon@webrtc.org>
> Reviewed-by: Björn Terelius <terelius@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#25848}

TBR=eladalon@webrtc.org,terelius@webrtc.org,zstein@webrtc.org,qingsi@webrtc.org,jeroendb@webrtc.org

Change-Id: Ib3b0a845f2300f4fcba2061650e17522735f08b3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9972
Reviewed-on: https://webrtc-review.googlesource.com/c/112581
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25852}

p2p/base/stunrequest.h

@@ -100,11 +100,6 @@ class StunRequest : public rtc::MessageHandler {
   // Returns the transaction ID of this request.
   const std::string& id() { return msg_->transaction_id(); }
 
-  // Returns the reduced transaction ID of this request.
-  uint32_t reduced_transaction_id() const {
-    return msg_->reduced_transaction_id();
-  }
-
   // the origin value
   const std::string& origin() const { return origin_; }
   void set_origin(const std::string& origin) { origin_ = origin; }