Delete Encounter API

## Feature description and context
Now that we've got a successful Create Encounter API (#826 ) as well as the ability to read one (#596 ), we need to fill out the CR**UD**. Delete is the next stage in the process.

## Feature sign-off requirements
API that successfully deletes an encounter.
* 200: "Successfully deleted encounter: {UUID}"
* 400: "Could not delete encounter: {UUID}"
