Remove unused QUIC version code This CL removes CurrentSupportedTransportVersions() and FilterSupportedTransportVersions() because they were no longer used apart from test code. This CL also improves the mechanism used by gfe_quic_fuzzer to enable all versions. gfe-relnote: remove unused code, no behavior change PiperOrigin-RevId: 284274377 Change-Id: I870ad7b202565281136f9d29ee9448c891d30512