Fix all no-unused-vars ESLint rule infractions (#1003)

DEFRA/water-abstraction-team#115

Following on from [Fix all max-len ESLint rule infractions](#1001) this change fixes the remaining infractions based on our current rules (all 1 of them!)

Author: Cruikshanks

test/services/return-requirements/check.service.test.js

@@ -14,7 +14,6 @@ const SessionHelper = require('../../support/helpers/session.helper.js')
 
 // Thing under test
 const CheckService = require('../../../app/services/return-requirements/check.service.js')
-const SessionModel = require('../../../app/models/session.model.js')
 
 describe('Return Requirements - Check service', () => {
   let session