chore: Removed unused express5 feature flag

lib/feature_flags.js

@@ -7,7 +7,10 @@
 
 // unreleased flags gating an active feature
 exports.prerelease = {
-  express5: false,
+  // internal_test_only is used for testing our feature flag implementation.
+  // It is not used to gate any features.
+  internal_test_only: false,
+
   promise_segments: false,
   reverse_naming_rules: false,
   undici_async_tracking: true,

test/smoke/index/index.tap.js

@@ -25,7 +25,7 @@ test('loading the application via index.js', { timeout: 15000 }, (t) => {
   let metric = agent.metrics.getMetric('Supportability/Nodejs/FeatureFlag/await_support/enabled')
   t.notOk(metric, 'should not create metric for unchanged feature flags')
 
-  metric = agent.metrics.getMetric('Supportability/Nodejs/FeatureFlag/express5/enabled')
+  metric = agent.metrics.getMetric('Supportability/Nodejs/FeatureFlag/internal_test_only/enabled')
   t.ok(metric, 'should create metric for changed feature flags')
 
   function shutdown() {

test/smoke/newrelic.js

@@ -23,7 +23,7 @@ exports.config = {
     enabled: true
   },
   feature_flag: {
-    await_support: false,
-    express5: true
+    internal_test_only: true,
+    await_support: false
   }
 }

test/unit/feature_flag.test.js

@@ -11,12 +11,13 @@ const Config = require('../../lib/config')
 
 // please do not delete flags from here
 const used = [
+  'internal_test_only',
+
   'async_local_context',
   'await_support',
   'cat',
   'custom_instrumentation',
   'custom_metrics',
-  'express5',
   'express_segments',
   'legacy_context_manager',
   'native_metrics',