Delete unused test class DummyPeerConnection

Introduced in https://webrtc-review.googlesource.com/c/src/+/152820
but never used.

Bug: None
Change-Id: I8888db9e0cad39a6eb073e8f662b5ad2690d03e5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/265409
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37173}

api/test/compile_all_headers.cc

@@ -27,7 +27,6 @@
 // "api/test/videocodec_test_fixture.h"
 // "api/test/videocodec_test_stats.h"
 
-#include "api/test/dummy_peer_connection.h"
 #include "api/test/fake_frame_decryptor.h"
 #include "api/test/fake_frame_encryptor.h"
 #include "api/test/mock_async_dns_resolver.h"