Drop user_canonical_emails table

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/848

This MR drops the user_canonical_emails table. The associated application code was removed in !167687 (merged)

Migration output:

▶ bin/rails db:migrate
main: == [advisory_lock_connection] object_id: 130080, pg_backend_pid: 2004116
main: == 20241021163518 DropUserCanonicalEmailsTable: migrating =====================
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- remove_foreign_key(:user_canonical_emails, {:column=>:user_id})
main:    -> 0.0073s
main: -- drop_table(:user_canonical_emails)
main:    -> 0.0050s
main: == 20241021163518 DropUserCanonicalEmailsTable: migrated (0.0542s) ============

main: == [advisory_lock_connection] object_id: 130080, pg_backend_pid: 2004116

▶ bundle exec rake db:migrate:down:main VERSION=20241021163518
main: == [advisory_lock_connection] object_id: 129860, pg_backend_pid: 2006549
main: == 20241021163518 DropUserCanonicalEmailsTable: reverting =====================
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- create_table(:user_canonical_emails)
main:    -> 0.0132s
main: -- add_index(:user_canonical_emails, [:user_id, :canonical_email], {:unique=>true})
main:    -> 0.0009s
main: -- add_index(:user_canonical_emails, :user_id, {:unique=>true})
main:    -> 0.0007s
main: == 20241021163518 DropUserCanonicalEmailsTable: reverted (0.0320s) ============

main: == [advisory_lock_connection] object_id: 129860, pg_backend_pid: 2006549

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