Remove special parameter ordering for contentType

[JoshLove-msft]

We have special ordering in RestClientProvider for contentType parameter, but this assumes that the contentType is optional. The contentType could be required. I'm not sure if the intent was to make the contentType the first optional parameter, in which case we could consider making it the first required parameter (if required), but there doesn't seem to be a good reason to not just use the spec-defined ordering.