fix: remove window.stop() from page.setContent() implementation (#26886)

The call was added back in 2019 to stop network loading. See commit:
56a48559c2

However, there's no evidence that this call is needed any more:
- all the tests pass without it
- `window.stop()` behavior is poorly defined, so relying on it is
unfortunate.

The `window.stop()` call, however, causes trouble while rolling firefox:
under certain condititions, the call prevents document from firing the
`load` event in the `document.open().write(..).close()` sequence that
comes immediately after the call. While this does look like a bug in
Firefox itself, we failed to reproduce it in isolation.

For the reference, the following tests fail with the Firefox 116 (using
`PWTEST_TRACE=1` triggers the race condition somewhere):

```bash
PWTEST_TRACE=1 npm run ftest cli-codegen
```

packages/playwright-core/src/server/frames.ts

@@ -896,7 +896,6 @@ export class Frame extends SdkObject {
           });
         });
         const contentPromise = context.evaluate(({ html, tag }) => {
-          window.stop();
           document.open();
           console.debug(tag);  // eslint-disable-line no-console
           document.write(html);