(core) Remove dead code for fetching snapshots

Summary: Deletes code which was previously only used by SharedSharing.ts, which was deleted in D2894

Test Plan: no

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2960

app/server/lib/ActiveDoc.ts

@@ -1049,11 +1049,6 @@ export class ActiveDoc extends EventEmitter {
     return sandboxActionBundle;
   }
 
-  public async fetchSnapshot() {
-    await this.waitForInitialization();
-    return this._pyCall('fetch_snapshot');
-  }
-
   // Needed for test/server/migrations.js tests
   public async testGetVersionFromDataEngine() {
     return this._pyCall('get_version');