chore(EventSource): drop no longer needed legacy fallback

[joshtrichards]

• Resolves: #

Summary

The OC\EventSource class has a legacy iframe-based fallback (dating back to ownCloud/IE9 era).

The workaround was needed back in 2015 because older Microsoft browsers were in an awkward in-between state:

• They began enforcing Content Security Policy (CSP) restrictions.
• But they still lacked proper EventSource (Server-Sent Events) support.

However:

• The EventSource API has been supported in all browsers since IE was retired (Chrome 6+, Firefox 6+, Safari 5+, Edge 12+)
• Nextcloud's own browser support requirements have long excluded IE
• The only consumer is the updater (UpdaterAdmin.vue -> new OCEventSource(...)) which will work identically with native SSE
• The useFallBack property defaults to false, so on any modern browser the fallback was already never triggered

TODO

• Follow-up (separate PR): Handle reconnect UX behavior better in Updater/UpdaterAdmin
  • fix(updater): Prevent EventSource auto-reconnect from triggering concurrent upgrades #59143
• Follow-up: Explore deprecating then eliminating eventsource.js / OC.EventSource outright (only seems to be a trivial wrapper over native EventSource + CSRF token and custom data object handling)
  • Could probably drop use by UpdaterAdmin.vue easily

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[joshtrichards]

/compile

[susnux]

Agreed! Thank you very much.

[szaimen]

🐘