[System.Xml] Fix deserialization issue (#16965)
[This ](https://github.com/mono/mono/pull/11194/files#diff-ae54d9565e735005686191de0f19ed50R431-R435)change in `XmlTypeMapElementInfo.GetElement` introduced the logic for handling the case of derived types deserialization.
For some cases (like https://github.com/mono/mono/issues/16918) it breaks existing functionality because it returns wrong element for given element name after searching withing derived types collection.
![image](https://user-images.githubusercontent.com/
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Unfortunately, what I can do here is just reverting the related changes.
Fixes https://github.com/mono/mono/issues/16918