Removed unused locale fields

refs https://github.com/TryGhost/Ghost/pull/12599

- These fields have been deprecated since API v2 and no longer are returned or exposed

ghost/admin/app/models/post.js

@@ -92,7 +92,6 @@ export default Model.extend(Comparable, ValidationEngine, {
     twitterDescription: attr('string'),
     emailSubject: attr('string'),
     html: attr('string'),
-    locale: attr('string'),
     visibility: attr('string'),
     metaDescription: attr('string'),
     metaTitle: attr('string'),

ghost/admin/app/models/user.js

@@ -20,7 +20,6 @@ export default BaseModel.extend(ValidationEngine, {
     location: attr('string'),
     accessibility: attr('string'),
     status: attr('string'),
-    locale: attr('string'),
     metaTitle: attr('string'),
     metaDescription: attr('string'),
     lastLoginUTC: attr('moment-utc'),

ghost/admin/mirage/factories/post.js

@@ -12,7 +12,6 @@ export default Factory.extend({
     featured: false,
     featureImage(i) { return `/content/images/2015/10/post-${i}.jpg`; },
     html(i) { return `<p>HTML for post ${i}.</p>`; },
-    locale: null,
     visibility: 'public',
     metaDescription(i) { return `Meta description for post ${i}.`; },
     metaTitle(i) { return `Meta Title for post ${i}`; },