chore(open-periods): Delete GroupOpenPeriod.event_id

[wedamija]

Final migration to fully delete the column

[sentry]

Bug: The migrations_lockfile.txt references migration 1027_delete_group_open_period_event_id, but the corresponding migration file is missing from the repository, which will cause the CI check to fail.
_{Severity: CRITICAL}

Suggested Fix

Add the missing migration file for 1027_delete_group_open_period_event_id. This file should contain the DeletionAction.DELETE operation, which is the second step of the SafeRemoveField pattern initiated in the preceding migration.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: migrations_lockfile.txt#L34

Potential issue: The `migrations_lockfile.txt` has been updated to reference migration
`1027_delete_group_open_period_event_id`, but the corresponding migration file is not
present in the repository. The CI workflow runs a script that validates the lockfile
against the actual migration files on disk using `sentry django makemigrations --check`.
This check will fail because the referenced migration file does not exist, which will
block the pull request from being merged and prevent deployment.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/1026_remove_group_open_period_event_id.py src/sentry/migrations/1027_delete_group_open_period_event_id.py

for 1026_remove_group_open_period_event_id in sentry

--
-- Moved groupopenperiod.event_id field to pending deletion state
--
-- (no-op)

for 1027_delete_group_open_period_event_id in sentry

--
-- Remove field event_id from groupopenperiod
--
ALTER TABLE "sentry_groupopenperiod" DROP COLUMN "event_id" CASCADE;