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
diff --git a/quic/core/quic_dispatcher.cc b/quic/core/quic_dispatcher.cc
index a1adda4..ae82fed 100644
--- a/quic/core/quic_dispatcher.cc
+++ b/quic/core/quic_dispatcher.cc
@@ -239,6 +239,8 @@
       expected_server_connection_id_length_(
           expected_server_connection_id_length),
       should_update_expected_server_connection_id_length_(false) {
+  QUIC_BUG_IF(GetSupportedVersions().empty())
+      << "Trying to create dispatcher without any supported versions";
   QUIC_DLOG(INFO) << "Created QuicDispatcher with versions: "
                   << ParsedQuicVersionVectorToString(GetSupportedVersions());
 }