Remove redundant foreign_key_exists? check in LFK script

What does this MR do and why?

This MR removes the redundant foreign_key_exists? check in the LFK script because remove_foreign_key_if_exists is called right below and this method does the same check internally.

Script generation works and migrations run successfully after this change.

Migration generated by script:

class RemoveProjectsPagesDeploymentsProjectIdFk < Gitlab::Database::Migration[2.1]
  disable_ddl_transaction!

  FOREIGN_KEY_NAME = "fk_rails_993b88f59a"

  def up
    with_lock_retries do
      remove_foreign_key_if_exists(:pages_deployments, :projects,
        name: FOREIGN_KEY_NAME, reverse_lock_order: true)
    end
  end

  def down
    add_concurrent_foreign_key(:pages_deployments, :projects,
      name: FOREIGN_KEY_NAME, column: :project_id,
      target_column: :id, on_delete: :cascade)
  end
end

Migration output

main: == [advisory_lock_connection] object_id: 249100, pg_backend_pid: 5778
main: == 20230828123652 RemoveProjectsPagesDeploymentsProjectIdFk: migrating ========
main: -- transaction_open?()
main:    -> 0.0000s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- execute("LOCK TABLE projects, pages_deployments IN ACCESS EXCLUSIVE MODE")
main:    -> 0.0004s
main: -- remove_foreign_key(:pages_deployments, :projects, {:name=>"fk_rails_993b88f59a"})
main:    -> 0.0074s
main: == 20230828123652 RemoveProjectsPagesDeploymentsProjectIdFk: migrated (0.0257s)

main: == [advisory_lock_connection] object_id: 249100, pg_backend_pid: 5778
ci: == [advisory_lock_connection] object_id: 269540, pg_backend_pid: 5780
ci: == 20230828123652 RemoveProjectsPagesDeploymentsProjectIdFk: migrating ========
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- execute("LOCK TABLE projects, pages_deployments IN ACCESS EXCLUSIVE MODE")
ci:    -> 0.0013s
ci: -- remove_foreign_key(:pages_deployments, :projects, {:name=>"fk_rails_993b88f59a"})
ci:    -> 0.0099s
ci: == 20230828123652 RemoveProjectsPagesDeploymentsProjectIdFk: migrated (0.0386s)

ci: == [advisory_lock_connection] object_id: 269540, pg_backend_pid: 5780

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