Remove download_code column from member_roles

What does this MR do and why?

We started ignoring column in !106256 (merged) (15.7). This MR removes it.

Related doc: https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-the-column-release-m1

Follow-up: #385222 (closed)

Database migration output

Migrate

main: == 20230319105436 RemoveMemberRoleDownloadCode: migrating =====================
main: -- remove_column(:member_roles, :download_code, :boolean, {:default=>false})
main:    -> 0.0122s
main: == 20230319105436 RemoveMemberRoleDownloadCode: migrated (0.0378s) ============

Rollback

main: == 20230319105436 RemoveMemberRoleDownloadCode: reverting =====================
main: -- add_column(:member_roles, :download_code, :boolean, {:default=>false})
main:    -> 0.0108s
main: == 20230319105436 RemoveMemberRoleDownloadCode: reverted (0.0281s) ============

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #385221 (closed)