Remove DiffNotesImporter and single_endpoint_notes_import branching

What does this MR do and why?

This MR completes the second phase of removing the "Use alternative comments import method" option from the GitHub importer. Phase 1 (GitLab 19.4, !250959 (merged)) removed the UI checkbox and hardcoded all new imports to use the single-endpoint diff notes import method, while preserving the old DiffNotesImporter class and related branching logic for production safety. With phase 1 now stabilized, this MR removes that dead code: the DiffNotesImporter class and its spec, the forced single_endpoint_notes_import setting in Settings#write, and the conditional branches in IssuableFinder#timeout and ImportIssuesAndDiffNotesWorker#importers.

All GitHub imports now consistently use the single-endpoint diff notes import method and the longer cache timeout, regardless of when they were originally configured.

References

Related to #617148 (closed) Phase 1: !250959 (merged)

Screenshots or screen recordings

N/A, backend-only change with no UI impact.

How to set up and validate locally

1. Start a GitHub project import (any project, no special configuration needed).
2. Confirm the import completes and diff notes/comments are imported successfully.
3. Confirm Gitlab::GithubImport::Importer::DiffNotesImporter no longer exists (bin/rails runner 'puts defined?(Gitlab::GithubImport::Importer::DiffNotesImporter)' prints nothing).

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.