Removed unused isPaymentConfigured method

refs https://github.com/TryGhost/Ghost/pull/11499

- Removed unused and confusin isPaymentConfigured because it was basing it's logic on old `isPaid` flag. Having it in the codebase was adding confusion.
- `isPaid` config flag still needs a proper cleanup with a migration etc.
- Added little post PR merge cleanup

core/frontend/helpers/ghost_head.js

@@ -50,7 +50,7 @@ function getMembersHelper() {
     const stripeSecretToken = stripePaymentProcessor.config.secret_token;
     const stripePublicToken = stripePaymentProcessor.config.public_token;
 
-    var membersHelper = `<script defer src="${getAssetUrl('public/members.js')}"></script>`;
+    let membersHelper = `<script defer src="${getAssetUrl('public/members.js')}"></script>`;
     if (!!stripeSecretToken && stripeSecretToken !== '' && !!stripePublicToken && stripePublicToken !== '') {
         membersHelper += '<script src="https://js.stripe.com/v3/"></script>';
     }

core/server/services/members/index.js

@@ -23,11 +23,6 @@ common.events.on('settings.edited', function updateSettingFromModel(settingModel
 });
 
 const membersService = {
-    isPaymentConfigured() {
-        const settings = settingsCache.get('members_subscription_settings');
-        return !!settings && settings.isPaid && settings.paymentProcessors.length !== 0;
-    },
-
     contentGating: require('./content-gating'),
 
     config: require('./config'),