[csharp-netcore] Better handling of oneOf/anyOf with duplicated data type

[wing328]

• Better handling of oneOf/anyOf with duplicated data type
• add tests
• remove skipOneOfAnyOfGetter option as it's no longer needed

to fix #14350

cc @OpenAPITools/generator-core-team

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cc @mandrean (2017/08) @frankyjuang (2019/09) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05)

[devhl-labs]

Here is a pr to this branch that will remove the extra spaces and also fixes a serialization issue.
#15413

[wing328]

merged :)