Remove now useless QUIC method AdjustErrorForVersion() which has been a no-op for some time.

net/quic/core/quic_protocol.h

Index: net/quic/core/quic_protocol.h
diff --git a/net/quic/core/quic_protocol.h b/net/quic/core/quic_protocol.h
index 3f8dbff1aed56fd9c8ed64753f40054332663efd..2f76cf53d649d40da0456b4e73e835142d7007e7 100644
--- a/net/quic/core/quic_protocol.h
+++ b/net/quic/core/quic_protocol.h
@@ -489,12 +489,6 @@ static_assert(static_cast<int>(QUIC_STREAM_LAST_ERROR) <=
                   std::numeric_limits<uint8_t>::max(),
               "QuicErrorCode exceeds single octet");
 
-// Because receiving an unknown QuicRstStreamErrorCode results in connection
-// teardown, we use this to make sure any errors predating a given version are
-// downgraded to the most appropriate existing error.
-NET_EXPORT_PRIVATE QuicRstStreamErrorCode
-AdjustErrorForVersion(QuicRstStreamErrorCode error_code, QuicVersion version);
-
 // These values must remain stable as they are uploaded to UMA histograms.
 // To add a new error code, use the current value of QUIC_LAST_ERROR and
 // increment QUIC_LAST_ERROR.