Avoid memory leak by allowing to release Style objects (bis)

Fixes #2092

src/PhpSpreadsheet/Spreadsheet.php

@@ -506,6 +506,7 @@ class Spreadsheet
         $this->disconnectWorksheets();
         $this->calculationEngine = null;
         $this->cellXfCollection = [];
+        $this->cellStyleXfCollection = [];
     }
 
     /**