Deletes a variable from a documents variable dictionary.
Ensure that the variable is no longer used by the queries in the document.
DELETE /documents/<documentID>/variables/<variableID>
Response type: application/xml or application/json
The response is a message stating the success or failure of the request.
DELETE /documents/8022/variables/L9
<success> <message>The resource of type "Variable" with identifier "L9" has been successfully deleted.</message> <id>L9</id> </success>