fix: remove the fragmentation handling stranded by #1303

#1298 and #1303 merged together. #1298 taught the creation paths to
resolve a fragmentation_level default; #1303 removed everything that
consumed it. Neither conflicted textually, so main ended up validating
the field on create and update, copying it on duplicate, resolving
default_fragmentation_level for it, and advertising it in the v1 API
docs — for a value nothing reads and a setting the Image Security tab no
longer exposes.

Inert rather than broken, which is exactly why it needed removing on
purpose: dead code that contradicts the PR that just deleted the feature
is how the next reader concludes fragmentation still works.

The events.fragmentation_level column and the app_settings row stay, as
#1303 decided — dropping a column is irreversible and the stored values
are harmless once nothing reads them.

Refs #1300

backend/__tests__/routes/adminEvents.smoke.test.js

@@ -224,7 +224,6 @@ describe('admin events CRUD endpoints (smoke)', () => {
       ['image_quality', [72]],
       ['protection_level', ['basic']],
       ['use_canvas_rendering', [false]],
-      ['fragmentation_level', [3]],
       // Not a protection field: the guard is not scoped to that block.
       ['event_name', ['Arrayed']],
       ['allow_downloads', [false]],