False positive redundant SerializeField attribute warning

With the change to make the "remove redundant attribute" fix a bulk fix that is also applied during code completion, any `[SerializeField]` usage on a non-serializable base class that has derived, serializable classes, should not be marked as redundant - code cleanup, or the bulk fix will incorrectly remove this attribute, and produce broken code.

This issue is already reported at [RIDER-18729](https://youtrack.jetbrains.com/issue/RIDER-18729), but also added here for visibility.