refactor(docs-infra): remove redundant overrides in FakeWebContainer

This is required to fix
```
adev/shared-docs/testing/testing-helper.ts:100:12 - error TS2416: Property 'on' in type 'FakeWebContainer' is not assignable to the same property in base type 'WebContainer'.
  Type '{ (event: "port", listener: PortListener): Unsubscribe; (event: "server-ready", listener: ServerReadyListener): Unsubscribe; (event: "error", listener: ErrorListener): Unsubscribe; (event: "preview-message", listener: PreviewMessageListener): Unsubscribe; }' is not assignable to type '{ (event: "port", listener: PortListener): Unsubscribe; (event: "server-ready", listener: ServerReadyListener): Unsubscribe; (event: "preview-message", listener: PreviewMessageListener): Unsubscribe; (event: "error", listener: ErrorListener): Unsubscribe; (event: "xdg-open", listener: OpenListener): Unsubscribe; (ev...'.
    Types of parameters 'event' and 'event' are incompatible.
      Type '"xdg-open"' is not assignable to type '"port"'.

100   override on(event: 'server-ready', listener: ServerReadyListener): Unsubscribe;
               ~~
```

See: https://github.com/angular/angular/actions/runs/14098789522/job/39491059550?pr=60526

Author: alan-agius4

adev/shared-docs/testing/testing-helper.ts

@@ -96,13 +96,11 @@ export class FakeWebContainer extends WebContainer {
 
     return Promise.resolve(fakeProcess);
   }
-  override on(event: 'port', listener: PortListener): Unsubscribe;
-  override on(event: 'server-ready', listener: ServerReadyListener): Unsubscribe;
-  override on(event: 'error', listener: ErrorListener): Unsubscribe;
-  override on(event: 'preview-message', listener: PreviewMessageListener): Unsubscribe;
+
   override on(event: unknown, listener: unknown): Unsubscribe {
     return () => {};
   }
+
   override mount(
     tree: FileSystemTree,
     options?: {mountPoint?: string | undefined} | undefined,