Remove unused NetworkRequest.durationDisplay getter and test (#9992)

Removes durationDisplay from NetworkRequest and its corresponding unit test in network_model_test.dart. The getter was not used anywhere in DevTools production code (durations are formatted directly via durationText in DurationColumn and HttpRequestOverviewView), which was causing DCM's check-unused-code to fail with 'getter durationDisplay is used only in tests'.

packages/devtools_app/lib/src/screens/network/network_model.dart

diff --git a/packages/devtools_app/lib/src/screens/network/network_model.dart b/packages/devtools_app/lib/src/screens/network/network_model.dart
index 76af3a9..c5a918c 100644
--- a/packages/devtools_app/lib/src/screens/network/network_model.dart
+++ b/packages/devtools_app/lib/src/screens/network/network_model.dart
@@ -41,15 +41,6 @@
 
   String get id;
 
-  @visibleForTesting
-  String get durationDisplay {
-    final duration = this.duration;
-    final text = duration != null
-        ? durationText(duration, unit: DurationDisplayUnit.milliseconds)
-        : 'Pending';
-    return 'Duration: $text';
-  }
-
   @override
   bool matchesSearchToken(RegExp regExpSearch) {
     return uri.caseInsensitiveContains(regExpSearch);

packages/devtools_app/test/screens/network/network_model_test.dart

diff --git a/packages/devtools_app/test/screens/network/network_model_test.dart b/packages/devtools_app/test/screens/network/network_model_test.dart
index 42d9396..f958186 100644
--- a/packages/devtools_app/test/screens/network/network_model_test.dart
+++ b/packages/devtools_app/test/screens/network/network_model_test.dart
@@ -415,15 +415,6 @@
       expect(httpWsHandshake.port, 56744);
     });
 
-    test('durationDisplay returns correct value', () {
-      expect(httpGet.durationDisplay, 'Duration: 811.7 ms');
-      expect(httpGetWithError.durationDisplay, 'Duration: 2029.5 ms');
-      expect(httpPost.durationDisplay, 'Duration: 1508.0 ms');
-      expect(httpPut.durationDisplay, 'Duration: 1325.8 ms');
-      expect(httpPatch.durationDisplay, 'Duration: 1243.7 ms');
-      expect(httpWsHandshake.durationDisplay, 'Duration: 41.4 ms');
-    });
-
     test('isValid returns correct value', () {
       expect(httpGet.isValid, isTrue);
       expect(httpGetWithError.isValid, isTrue);