Only clean up on Capacity change

Author: slozier

src/core/IronPython/Runtime/Types/PythonType.cs

@@ -2498,7 +2498,9 @@ private void AddSubType(PythonType subtype) {
             }
 
             lock (_subtypesLock) {
-                _subtypes.RemoveAll(x => !x.TryGetTarget(out _)); // remove dead entries
+                if (_subtypes.Count == _subtypes.Capacity) {
+                    _subtypes.RemoveAll(x => !x.TryGetTarget(out _)); // remove dead entries
+                }
                 _subtypes.Add(new WeakReference<PythonType>(subtype));
             }
         }