Adds one entry point for both JSON and MP APIs

Removes endpoints folder and moves versions to the root
JSON API folder

lib/API/MP/API.php

@@ -1,15 +0,0 @@
-<?php
-namespace MailPoet\MP\API;
-
-if(!defined('ABSPATH')) exit;
-
-class API {
-  static function __callStatic($version, $arguments) {
-    $api_class = sprintf('%s\%s\API', __NAMESPACE__, $version);
-    if(class_exists($api_class)) {
-      $api = new $api_class();
-      return $api;
-    }
-    throw new \Exception(__('Invalid API version.', 'mailpoet'));
-  }
-}