Remove unused ninon sandbox from settings unit tests

Author: tanx

test/unit/action/setting.spec.js

@@ -8,20 +8,14 @@ describe('Action Setting Unit Test', () => {
   let wallet;
   let db;
   let setting;
-  let sandbox;
 
   beforeEach(() => {
-    sandbox = sinon.createSandbox({});
     store = new Store();
     wallet = sinon.createStubInstance(WalletAction);
     db = sinon.createStubInstance(AppStorage);
     setting = new SettingAction(store, wallet, db);
   });
 
-  afterEach(() => {
-    sandbox.restore();
-  });
-
   describe('setBitcoinUnit()', () => {
     it('should set a valid unit and save settings', () => {
       setting.setBitcoinUnit({ unit: 'sat' });