Remove alternative comments import method checkbox from GitHub importer

What does this MR do and why?

Removes the "Use alternative comments import method" checkbox from the GitHub importer UI, and hardcodes Gitlab::GithubImport::Settings#write so every newly configured GitHub import has single_endpoint_notes_import enabled -- the slower but reliable comments import method that avoids GitHub API pagination limits, previously opt-in and off by default.

This is phase 1 of a two-phase rollout for work item #617148, done this way deliberately for production safety:

• Phase 1 (this MR): remove the UI option and force it on for all new imports via Settings#write. The change is write-time only, so any project's already-persisted import_data keeps whatever it was previously configured with -- in-progress imports are unaffected. The old DiffNotesImporter code path and the worker/issuable_finder branching that reads this setting are intentionally left in place.
• Phase 2 - next milestone (a separate follow-up MR): once phase 1 has baked in production, delete DiffNotesImporter and the now-permanently- dead branching entirely.

References

Related to #617148

How to set up and validate locally

1. Start a GitHub project import.
2. Confirm the "Use alternative comments import method" checkbox no longer appears in the import's advanced options.
3. Perform a migration
4. Diff notes should be imported successfully

MR acceptance checklist

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