[csharp][generichost] Remove discriminator property

[devhl-labs]

Please merge this first https://github.com/OpenAPITools/openapi-generator/pull/18408 closes https://github.com/OpenAPITools/openapi-generator/pull/17397

Removes the discriminator because new Whale("whale") is redundant. We can just do new Whale()

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[gerwinjansen]

Nice improvement, thanks! Just a new minor comments:

• in the generated code the discriminator constant is now spread across 2 classes. For instance, the constant "BasquePig" exists in Pig.Read() and BasquePig.Write(). Not a big deal because you don't have to maintain generated code.
• BasquePig.Read() also reads the discriminator, which is kind of duplicate on what Pig.Read() also does. Could be a (minor) performance improvement to skip it.

[devhl-labs]

Kept the discriminator property so de/serialization can still work with libraries other than STJ. Hard coded the value of the discriminator property so we can remove it from the constructor. Reworked the manual tests to compare json instead of classes to be more concise. Also added more manual tests to ensure de/serialization works. I think this is ready to go @wing328