Removes fennec migration code

[tarikeshaq]

It's been a long time coming, but this PR finally removes the fennec to fenix migration code.

I won't land this until I have the PRs in firefox-android to remove the code there... or I might opt to land the removal there first, so this becomes a not-breaking change.

NOTE: Althought fenix no longer runs the migration, the code is still alive in android components last I checked, which is why this is "breaking"

Regardless opening a PR to get some eyes on it. This was very mechanical!

Pull Request checklist

• Quality: This PR builds and tests run cleanly
  • Note:
    • For changes that need extra cross-platform testing, consider adding [ci full] to the PR title.
    • If this pull request includes a breaking change, consider cutting a new release after merging.
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry in CHANGES_UNRELEASED.md or an explanation of why it does not need one
  • Any breaking changes to Swift or Kotlin binding APIs are noted explicitly
• Dependencies: This PR follows our dependency management guidelines
  • Any new dependencies are accompanied by a summary of the due dilligence applied in selecting them.

Branch builds: add [ff-android: firefox-android-branch-name] and/or [fenix: fenix-branch-name] to the PR title.

[mhammond]

🚀

[tarikeshaq]

opened mozilla-mobile/firefox-android#245, will wait until that, or some variation of it lands before I land this - then this will be a non-breaking change

[codecov-commenter]

Codecov Report

Base: 44.30% // Head: 44.30% // No change to project coverage 👍

Coverage data is based on head (1ba86f3) compared to base (db3e2a9).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5268   +/-   ##
=======================================
  Coverage   44.30%   44.30%           
=======================================
  Files         175      175           
  Lines       13898    13898           
=======================================
  Hits         6158     6158           
  Misses       7740     7740           

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[lougeniaC64]

🚢

[tarikeshaq]

Change was landed in android-components, verified that building with this PR is not a breaking change!