DEV: Clean up helpers

* remove an unused helper
* move a single-use helper to its call-site as a getter
* convert `format-currency` to a pure function helper

Author: cvx

assets/javascripts/discourse/connectors/user-main-nav/billing.gjs

@@ -1,15 +1,25 @@
 import Component from "@ember/component";
 import { LinkTo } from "@ember/routing";
+import { service } from "@ember/service";
 import { classNames, tagName } from "@ember-decorators/component";
 import icon from "discourse/helpers/d-icon";
 import { i18n } from "discourse-i18n";
-import userViewingSelf from "../../helpers/user-viewing-self";
 
 @tagName("li")
 @classNames("user-main-nav-outlet", "billing")
 export default class Billing extends Component {
+  @service currentUser;
+
+  get viewingSelf() {
+    return (
+      this.currentUser &&
+      this.currentUser.username.toLowerCase() ===
+        this.model.username.toLowerCase()
+    );
+  }
+
   <template>
-    {{#if (userViewingSelf this.model)}}
+    {{#if this.viewingSelf}}
       <LinkTo @route="user.billing">
         {{icon "far-credit-card"}}
         {{i18n "discourse_subscriptions.navigation.billing"}}

assets/javascripts/discourse/helpers/format-currency.js

@@ -1,6 +1,4 @@
-import { helper } from "@ember/component/helper";
-
-export function formatCurrency([currency, amount]) {
+export default function formatCurrency(currency, amount) {
   let currencySign;
 
   switch (currency.toUpperCase()) {
@@ -38,8 +36,6 @@ export function formatCurrency([currency, amount]) {
       currencySign = "$";
   }
 
-  let formattedAmount = parseFloat(amount).toFixed(2);
+  const formattedAmount = parseFloat(amount).toFixed(2);
   return currencySign + formattedAmount;
 }
-
-export default helper(formatCurrency);

assets/javascripts/discourse/helpers/stripe-payment-link.js

@@ -1,17 +1,17 @@
 import { setupTest } from "ember-qunit";
 import { module, test } from "qunit";
-import { formatCurrency } from "discourse/plugins/discourse-subscriptions/discourse/helpers/format-currency";
+import formatCurrency from "discourse/plugins/discourse-subscriptions/discourse/helpers/format-currency";
 
 module("Subscriptions | Unit | Helper | format-currency", function (hooks) {
   setupTest(hooks);
 
-  test("it formats USD correctly", function (assert) {
-    let result = formatCurrency(["USD", 338.2]);
+  test("formats USD correctly", function (assert) {
+    const result = formatCurrency("USD", 338.2);
     assert.equal(result, "$338.20", "Formats USD correctly");
   });
 
-  test("it rounds correctly", function (assert) {
-    let result = formatCurrency(["USD", 338.289]);
+  test("rounds correctly", function (assert) {
+    const result = formatCurrency("USD", 338.289);
     assert.equal(result, "$338.29", "Rounds correctly");
   });
 });