Remove Equals and GetHashCode methods from AnyOf

[ob-stripe]

r? @brandur-stripe
cc @stripe/api-libraries

Remove the Equals and GetHashCode methods from the AnyOf wrapper class.

In C#, Equals and GetHashCode should be overridden to give value semantics to a reference type (cf. https://docs.microsoft.com/en-us/dotnet/api/system.object.equals?view=netframework-4.8).

The type held by an AnyOf instance may be either a value type (e.g. int, bool, etc.) or a reference type (e.g. DateRangeOptions, CardCreateNestedOptions, etc.). By delegating Equals to the value held within the AnyOf instance, we're implying that the value has value semantics even when that's not the case.

This breaks assumptions in some testing frameworks like Fluent Assertions, as they use reflection to see if a type overrides Equals and assume that it has value semantics if that's the case (cf. https://www.continuousimprover.com/2018/02/fluent-assertions-50-best-unit-test.html).

The library itself doesn't rely on AnyOf overriding Equals anywhere, so let's just remove it.

I did leave the ToString method that delegates to the held value since that seems useful for debugging (e.g. to log the values of request parameters).

[brandur-stripe]

Thanks for the detailed explanation! Very helpful.