Makes the `FLAGS_` prefix implicit in calls to `GetQuicFlag()`.

This refactoring will allow us to define separate sets of flags for QUICHE in google3 vs //third_party/envoy/external_quiche. This will be necessary to import upstream Envoy PR [#22907](https://github.com/envoyproxy/envoy/pull/22907).

CL automatically created by:
```
    replace_string \
      'GetQuicFlag\(FLAGS_' \
      'GetQuicFlag('

    replace_string \
      'SetQuicFlag\(FLAGS_' \
      'SetQuicFlag('
```

Tested:
```
  $ blaze test //third_party/quic/core/... //gfe/gfe2/quic:all //net/quic/... //third_party/castlite/agent/net/... //video/streaming/quic:all
  http://sponge2/e6f39d62-e455-451e-ae09-84ebb20806b8
```
PiperOrigin-RevId: 473802782
diff --git a/quiche/quic/core/quic_versions.cc b/quiche/quic/core/quic_versions.cc
index df43ce7..cc9065b 100644
--- a/quiche/quic/core/quic_versions.cc
+++ b/quiche/quic/core/quic_versions.cc
@@ -25,7 +25,7 @@
 
 QuicVersionLabel CreateRandomVersionLabelForNegotiation() {
   QuicVersionLabel result;
-  if (!GetQuicFlag(FLAGS_quic_disable_version_negotiation_grease_randomness)) {
+  if (!GetQuicFlag(quic_disable_version_negotiation_grease_randomness)) {
     QuicRandom::GetInstance()->RandBytes(&result, sizeof(result));
   } else {
     result = MakeVersionLabel(0xd1, 0x57, 0x38, 0x3f);