🏗 Removed author_id column from the posts table

refs https://github.com/TryGhost/Toolbox/issues/230

- This is a first step to removal of `author_id` concept from the codebase. The aim is to see what parts break down from this changes an work backwards from there

core/server/data/migrations/versions/5.0/2022-04-28-03-26-remove-author-id-column-from-posts-table.js

@@ -0,0 +1,7 @@
+const {createDropColumnMigration} = require('../../utils');
+
+module.exports = createDropColumnMigration('posts', 'author_id', {});
+
+module.exports.down = async () => {
+    // noop - major version migrations are not backwards compatible
+};

core/server/data/schema/schema.js

@@ -67,11 +67,6 @@ module.exports = {
             nullable: false,
             defaultTo: 'none'
         },
-        /**
-         * @deprecated: single authors was superceded by multiple authors in Ghost 1.22.0
-         * If we keep it, then only, because you can easier query post.author_id than posts_authors[*].sort_order.
-         */
-        author_id: {type: 'string', maxlength: 24, nullable: false},
         created_at: {type: 'dateTime', nullable: false},
         /**
          * @deprecated: https://github.com/TryGhost/Ghost/issues/10286