Remove WP\Functions::__ and other translate functions

Under the new sniffer rules, those functions produce errors and, when those methods
are used, the sniffer can not properly be applied.

[MAILPOET-4524]

mailpoet/lib/API/JSON/ErrorResponse.php

@@ -2,8 +2,6 @@
 
 namespace MailPoet\API\JSON;
 
-use MailPoet\WP\Functions as WPFunctions;
-
 class ErrorResponse extends Response {
   public $errors;
 
@@ -24,7 +22,7 @@ class ErrorResponse extends Response {
     return array_map(function($error, $message) {
       // sanitize SQL error
       if (preg_match('/^SQLSTATE/i', $message)) {
-        $message = WPFunctions::get()->__('An unknown error occurred.', 'mailpoet');
+        $message = __('An unknown error occurred.', 'mailpoet');
       }
       return [
         'error' => $error,