Fix issue with delete_order

[RichardAtCT]

TlDr: The _do method was not handling the body-less response resonse from VALRs API. To fix this I created a new method called do_delete which skips a lot of the error handling in _do, just returning the request object.

[RichardAtCT]

Hey @jonathanelscpt - have you had a chance to look at this? :)