chore: remove dead non multibackend code paths - WPB-12140

[netbe]

WPB-12140 [iOS] Clean up legacy BackendEnvironment code

Issue

This is one of the PRs removing multibackend dead code, this one focus on removing direct dead code:

• remove DeveloperFlag.multibackend
• SharingSession convenience initializers

Next:

• BackendEnvironment2 to be rename
• remove WireTransport.BackendEnvironment
• remove APIVersionResolver
• remove dead AuthenticationCoordinator cases

Testing

Describe how to test.

Optional: attachments like images, videos, etc.

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Checklist

• Title contains a reference JIRA issue number like [WPB-XXX].
• Description is filled and free of optional paragraphs.
• Adds/updates automated tests.

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UI accessibility checklist

If your PR includes UI changes, please utilize this checklist:

• Make sure you use the API for UI elements that support large fonts.
• All colors are taken from WireDesign.ColorTheme or constructed using WireDesign.BaseColorPalette.
• New UI elements have Accessibility strings for VoiceOver.

[github-actions]

Test Results

    4 files    474 suites   3m 56s ⏱️
3 249 tests 3 221 ✅ 27 💤 1 ❌
3 249 runs  3 222 ✅ 27 💤 0 ❌

For more details on these failures, see this check.

Results for commit bfb1534.

♻️ This comment has been updated with latest results.

[datadog-wireapp]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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