Remove the defunct DiagnosticUtility/Fx classes from DataContractSerialization (...

Remove the defunct DiagnosticUtility/Fx classes from DataContractSerialization (#82324)

- Fx.Assert just delegated to Debug.Assert: replace all the call sites with Debug.Assert/Fail.
- DiagnosticUtility.DebugAssert just delegated to Debug.Assert: replace all the call sites with Debug.Assert/Fail.
- DiagnosticUtility.ExceptionUtility.ThrowHelperError didn't throw anything and just returned its argument: removed all uses of it
- DiagnosticUtility.ExceptionUtility.ThrowHelperCallback didn't throw anything and just returned its argument: removed all uses of it.
- DiagnosticUtility.ExceptionUtility.ThrowHelperFatal didn't throw anything and just wrapped its arguments in a new Exception that was then returned: just changed the call sites to create that exception.
- DiagnosticUtility.ExceptionUtility.ThrowHelperArgument{Null} didn't throw anything and just returned new instances of Argument{Null}Exception: just changed the call sites to create that exception.
- Replaced some argument validation with helpers like ThrowIfNegative.
- Fx.IsFatal: moved to ExceptionUtility class