Firebase v9 loads a large iframe.js file but only on mobile

### [REQUIRED] Describe your environment

* Operating System version: macOS v11.3
* Browser version: Chrome 90.0.4430.212
* Firebase SDK version: firebase@9.0.0-beta.2
* Firebase Product: auth

### [REQUIRED] Describe the problem

After upgrading from v8 to v9 a request for an iframe (`https://[projectname].firebaseapp.com/__/auth/iframe.js`) has started appearing on every page load, but only on mobile (via Chrome's simulated Device mode at least, and when Lighthouse is auditing the website).

An older Lighthouse report of our application, from before the upgrade to v9, did not mention this file at all so I can only assume it wasn't loaded back then.

This iframe file is big and seems unnecessary to our application since we never use any kind of iframing of Firebase functionality and the only auth login method we use is email login.

Is there a way to disable the loading of this iframe?