🚀 Remove unnecessary MutationObserver fallback (#31421)
🚀 Remove unnecessary MutationObserver fallback (#31421)

All `module` browsers support `MutationObserver`.

https://caniuse.com/?search=MutationObserver

IE 11 also supports it, so `nomodule` builds should be fine too. We may then remove the fallback altogether, but some strictly unsupported outliers (like Opera Mini) would be left out:

```diff
-  if (IS_ESM || win.MutationObserver) {
    /** @const {MutationObserver} */
    const observer = new win.MutationObserver(() => {
      if (checkFunc(parent)) {
        observer.disconnect();
        callback();
      }
    });
    observer.observe(parent, {childList: true});
-  } else {
-     /** @const {number} */
-    const interval = win.setInterval(() => {
-      if (checkFunc(parent)) {
-        win.clearInterval(interval);
-        callback();
-      }
-    }, /* milliseconds */ 5);
-  }
```

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