Remove switch to classic from gutenberg

[guarani]

Addresses: wordpress-mobile/gutenberg-mobile#3048 (for iOS)

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 Android.

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

Regression notes (new ✨ )

I'm adding this section to try out @startuptester's proposal to improve code quality.

• Potential unintended areas of impact (regressions)
  The app has a couple of different editors (block editor, classic editor, story editor, HTML editor) and this change is only intended to prevent navigation from the block editor to the classic editor.

• What you did to manually test those/ what existing automated tests you relied on
  I gave these the four editors (block, classic, story, HTML) a quick test, keeping an eye out for any unintended side-effects of this change.

• What automated tests you added or what prevented you from doing so
  There are no existing UI tests nor unit tests that cover switching from the block editor to the classic editor, so there were no tests to update. Since we're removing functionality here, there's are no tests to add, either.

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 test the changes on this Pull Request by downloading it from AppCenter here with build number: 44237. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[peril-wordpress-mobile]

	Warnings
⚠️	This PR is tagged with 'DO NOT MERGE'.

Generated by 🚫 dangerJS

[peril-wordpress-mobile]

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

[guarani]

Tested build 44237:

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

Looks good 🟢 Submitting for review!

[illusaen]

Looks good! I <3 this change :)

[guarani]

Thanks @illusaen!