Fix deletion for the test-repo backend.

Deletion was added in #485, but the function for the `test-repo` backend
was `deleteEntry` instead of `deleteFile` like it was supposed to be.

Also, setting the key for a deleted file to `undefined` did not really
remove that file from the object, so there were then errors stating
`file.content` is not defined. `delete`ing the "file" from the object
fixes this bug.

src/backends/test-repo/implementation.js

@@ -93,10 +93,10 @@ export default class TestRepo {
     return Promise.resolve();
   }
 
-  deleteEntry(path, commitMessage) {
+  deleteFile(path, commitMessage) {
     const folder = path.substring(0, path.lastIndexOf('/'));
     const fileName = path.substring(path.lastIndexOf('/') + 1);
-    window.repoFiles[folder][fileName] = undefined;
+    delete window.repoFiles[folder][fileName];
     return Promise.resolve();
   }
 }