Chore: Remove dead code related to billing (#935)

- remove subscription model
- remove billing-related code

app/javascript/dashboard/store/modules/billing.js

@@ -1,60 +0,0 @@
-/* eslint no-console: 0 */
-/* eslint no-param-reassign: 0 */
-/* eslint no-shadow: 0 */
-import * as types from '../mutation-types';
-import Billing from '../../api/billing';
-
-const state = {
-  fetchingStatus: false,
-  billingDetails: {},
-  status: null,
-};
-
-const getters = {
-  getBillingDetails(_state) {
-    return _state.billingDetails;
-  },
-  billingFetchStatus(_state) {
-    return _state.fetchingStatus;
-  },
-};
-
-const actions = {
-  fetchSubscription({ commit }) {
-    commit(types.default.TOGGLE_SUBSCRIPTION_LOADING, true);
-    Billing.getSubscription()
-      .then(billingDetails => {
-        commit(types.default.SET_SUBSCRIPTION, billingDetails.data);
-        commit(
-          types.default.TOGGLE_SUBSCRIPTION_LOADING,
-          false,
-          billingDetails.status
-        );
-      })
-      .catch(error => {
-        const { response } = error;
-        commit(
-          types.default.TOGGLE_SUBSCRIPTION_LOADING,
-          false,
-          response.status
-        );
-      });
-  },
-};
-
-const mutations = {
-  [types.default.SET_SUBSCRIPTION](_state, billingDetails) {
-    _state.billingDetails = billingDetails;
-  },
-  [types.default.TOGGLE_SUBSCRIPTION_LOADING](_state, flag, apiStatus) {
-    _state.fetchingStatus = flag;
-    _state.status = apiStatus;
-  },
-};
-
-export default {
-  state,
-  getters,
-  actions,
-  mutations,
-};