Cleanup custom handling of Obsolete attribute once we get updates from STJ flowing

The PR manually adds deprecated property to the schema when ObsoleteAttribute is found on type/property.

This was recently implemented on JsonSchemaExporter side in dotnet/runtime#130665, but the runtime change hasn't flown to aspnetcore yet. Once it flows, we should try to cleanup the implementation on our side and rely on JsonSchemaExporter doing it.