chore(events): drop the dead coach_description column

[mroderick]

Part of #3019, stacked on the ignore PR (base = its head branch; when that merges, GitHub retargets this PR to master). Closes #3019 once merged after the ignore PR ships and soaks in production.

Removes coach_description from events with safety_assured { remove_column }, regenerates db/schema.rb, and drops the transient legacy-column specs.

Detail

Two commits mirroring the merged meetings and workshops drops: the migration plus schema regeneration, then the spec removal — each green on its own. The column holds 3 non-null values out of 260 production rows, all unread since the 2016-06-14 invitation mailer switch to event.description (de44fe17). The migration was regenerated from bin/rails db:migrate rather than edited by hand.

Review notes

• Merge order gate: ignore PR first, deploy, soak, then this one. The stacked base enforces the ordering in GitHub; the soak window is operational.
• Deliberately not done: no data migration or backfill — the three stale values go with the column.

[KimberleyCook]

I'm a big fan of this tidy up 🎉