Remove unused sinon sandboxes (#8063)

These sandboxes were created, then not utilized at all.

test/unit/app/controllers/infura-controller-test.js

@@ -3,21 +3,16 @@ import sinon from 'sinon'
 import InfuraController from '../../../../app/scripts/controllers/infura'
 
 describe('infura-controller', function () {
-  let infuraController, sandbox, networkStatus
+  let infuraController, networkStatus
   const response = { 'mainnet': 'degraded', 'ropsten': 'ok', 'kovan': 'ok', 'rinkeby': 'down', 'goerli': 'ok' }
 
   describe('Network status queries', function () {
     before(async function () {
       infuraController = new InfuraController()
-      sandbox = sinon.createSandbox()
       sinon.stub(infuraController, 'checkInfuraNetworkStatus').resolves(response)
       networkStatus = await infuraController.checkInfuraNetworkStatus()
     })
 
-    after(function () {
-      sandbox.restore()
-    })
-
     describe('Mainnet', function () {
       it('should have Mainnet', function () {
         assert.equal(Object.keys(networkStatus)[0], 'mainnet')

test/unit/reducers/unlock_vault_test.js

@@ -1,19 +1,8 @@
 import assert from 'assert'
-import sinon from 'sinon'
 import * as actions from '../../../ui/app/store/actions'
 import reducers from '../../../ui/app/ducks'
 
 describe('#unlockMetamask(selectedAccount)', function () {
-  beforeEach(function () {
-    // sinon allows stubbing methods that are easily verified
-    this.sinon = sinon.createSandbox()
-  })
-
-  afterEach(function () {
-    // sinon requires cleanup otherwise it will overwrite context
-    this.sinon.restore()
-  })
-
   describe('after an error', function () {
     it('clears warning', function () {
       const warning = 'this is the wrong warning'

ui/app/helpers/utils/util.test.js

@@ -1,5 +1,4 @@
 import assert from 'assert'
-import sinon from 'sinon'
 import ethUtil from 'ethereumjs-util'
 import * as util from './util'
 
@@ -9,14 +8,6 @@ describe('util', function () {
     ethInWei += '0'
   }
 
-  beforeEach(function () {
-    this.sinon = sinon.createSandbox()
-  })
-
-  afterEach(function () {
-    this.sinon.restore()
-  })
-
   describe('#parseBalance', function () {
     it('should render 0.01 eth correctly', function () {
       const input = '0x2386F26FC10000'