Simplify DtlsTransport state.

Make a few more members const, remove members that aren't used,
set max ssl version number on construction and remove setter.

Bug: none
Change-Id: I6c1a7cabf1e795e027f1bc53b994517e9aef0e93
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/213780
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33622}

pc/jsep_transport_controller_unittest.cc

@@ -57,7 +57,8 @@ class FakeDtlsTransportFactory : public cricket::DtlsTransportFactory {
  public:
   std::unique_ptr<cricket::DtlsTransportInternal> CreateDtlsTransport(
       cricket::IceTransportInternal* ice,
-      const webrtc::CryptoOptions& crypto_options) override {
+      const webrtc::CryptoOptions& crypto_options,
+      rtc::SSLProtocolVersion max_version) override {
     return std::make_unique<FakeDtlsTransport>(
         static_cast<cricket::FakeIceTransport*>(ice));
   }