Remove the sliding sync proxy force logout dialog
The SDK no longer exposes the sliding sync proxy version: `SlidingSyncVersion` maps
only to `None` or `Native`. So `ForceNativeSlidingSyncMigrationDialog` could never
be shown. iOS removed the same alert in element-hq/element-x-ios#4367.
Remove the dialog, its events and state from the LoggedIn screen. Also remove
`MatrixClient.currentSlidingSyncVersion()` and the `SlidingSyncVersion` type, which
are now unused.
The string resources are kept for now; they will be removed from Localazy later.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
