Hide namespace dropdown and instead utilize existing namespace details for add-on purchases

1. Pass the current_namespace_id and current_namespace_name sent from backend as props to AddonTransactionApp from init_addon_transaction.js.
2. Cleanup the code in addon_transaction_app.vue to remove the usage of selectedNamespaceId and instead use the currentNamespaceId.
3. Pass currentNamespaceName as a prop to purchase_details_card.vue.
4. In purchase_details_card.vue, replace the GlFormGroup with currentNamespaceName.
5. Add specs.
6. Remove the redundant namespaces data passed from _add_on.html.haml.