Delete the unused QuartcEndpointImpl class.

This is a leftover from a different arrangement of QuartcEndpoint internals.  It
is no longer needed.

gfe-relnote: n/a (Quartc only, dead code)
PiperOrigin-RevId: 257880931
Change-Id: I6689db09861d67a2a4ac470b5d921ab64e7e76ff

quic/quartc/quartc_endpoint.h

diff --git a/quic/quartc/quartc_endpoint.h b/quic/quartc/quartc_endpoint.h
index 51f35ce..0f557a2 100644
--- a/quic/quartc/quartc_endpoint.h
+++ b/quic/quartc/quartc_endpoint.h
@@ -18,15 +18,6 @@
 
 namespace quic {
 
-// Private implementation of QuartcEndpoint.  Enables different implementations
-// for client and server endpoints.
-class QuartcEndpointImpl {
- public:
-  virtual ~QuartcEndpointImpl() = default;
-
-  virtual QuicStringPiece server_crypto_config() const = 0;
-};
-
 // Endpoint (client or server) in a peer-to-peer Quartc connection.
 class QuartcEndpoint {
  public: