Removed settings populateDefaults pre-migration

refs https://github.com/TryGhost/Ghost/issues/12568
refs 5fbc40430b

- Having populateDefaults run before migrations creates a chicken and egg problem where populate defaults can create records that are "non-migratable" as happened in https://github.com/TryGhost/Ghost/issues/12026

core/server/data/migrations/hooks/migrate/before.js

@@ -3,8 +3,5 @@ const models = require('../../../../models');
 
 module.exports = function before() {
     models.init();
-    return dbBackup.backup().then(() => {
-        // ensure that our default settings are created to limit possible db states in migrations
-        return models.Settings.populateDefaults();
-    });
+    return dbBackup.backup();
 };