Remove workaround for fixed Roslyn bug in HttpHeaders (#110790)

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HttpHeaders.cs

@@ -334,6 +334,8 @@ public IEnumerator<KeyValuePair<string, IEnumerable<string>>> GetEnumerator() =>
 
         private IEnumerator<KeyValuePair<string, IEnumerable<string>>> GetEnumeratorCore()
         {
+            Debug.Assert(_headerStore is not null);
+
             HeaderEntry[]? entries = GetEntriesArray();
             Debug.Assert(_count != 0 && entries is not null, "Caller should have validated the collection is not empty");
 
@@ -347,14 +349,11 @@ private IEnumerator<KeyValuePair<string, IEnumerable<string>>> GetEnumeratorCore
                     // during enumeration so that we can parse the raw value in order to a) return
                     // the correct set of parsed values, and b) update the instance for subsequent enumerations
                     // to reflect that parsing.
-
-#nullable disable // https://github.com/dotnet/roslyn/issues/73928
                     ref object storeValueRef = ref EntriesAreLiveView
                         ? ref entries[i].Value
                         : ref CollectionsMarshal.GetValueRefOrNullRef((Dictionary<HeaderDescriptor, object>)_headerStore, entry.Key);
 
                     info = ReplaceWithHeaderStoreItemInfo(ref storeValueRef, entry.Value);
-#nullable restore
                 }
 
                 // Make sure we parse all raw values before returning the result. Note that this has to be