Remove switch to classic from gutenberg

[cameronvoell]

Addresses: wordpress-mobile/gutenberg-mobile#3048 (for Android)
See WPiOS PR: wordpress-mobile/WordPress-iOS#16131

Relevant info Borrowed from WPiOS PR:
In discussion with @kyleaparker, we decided that it's best to fast-track the removal of the "switch to classic" option in the block editor. This PR will be scheduled for release in v17.1 alongside a matching change for iOS.

I think this change doesn't warrant inclusion in RELEASE-NOTES.txt, but if anyone thinks differently I'd love to hear.

To test

1. Select a Simple site
2. Open a new post in the block editor
3. Tap the ellipsis button to reveal the options bottom sheet
4. Verify that there is no longer an option to switch to classic
5. Repeat Steps 2-4 for Atomic, Jetpack, self-hosted sites

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[guarani]

Tested on 783faf6 and looks good 🟢

1. Select a Simple site
2. Open a new post in the block editor
3. Tap the ellipsis button to reveal the options bottom sheet
4. Verify that there is no longer an option to switch to classic
5. Repeat Steps 2-4 for Atomic, Jetpack, self-hosted sites

(There's a mention of Android in the PR description that should say iOS)