Remove isomorphic-fetch dependency

__tests__/src/lib/MiradorViewer.test.js

@@ -4,7 +4,6 @@ import MiradorViewer from '../../../src/lib/MiradorViewer';
 
 jest.unmock('react-i18next');
 jest.mock('react-dom');
-jest.mock('isomorphic-unfetch', () => jest.fn(() => Promise.resolve({ json: () => ({}) })));
 
 describe('MiradorViewer', () => {
   let container;

package.json

@@ -43,7 +43,6 @@
     "dompurify": "^2.0.11",
     "i18next": "^19.5.0",
     "icomcom-react": "^1.0.1",
-    "isomorphic-unfetch": "^3.0.0",
     "jss": "^10.3.0",
     "jss-rtl": "^0.3.0",
     "lodash": "^4.17.11",