Fix fatal error on mongodb driver

Fatal error: phpFastCache\Drivers\Mongodb\Driver and phpFastCache\Core\Pool\DriverBaseTrait define the same property ($instance) in the composition of phpFastCache\Drivers\Mongodb\Driver. However, the definition differs and is considered incompatible.

Author: jomisacu

src/phpFastCache/Drivers/Mongodb/Driver.php

@@ -33,11 +33,6 @@
  */
 class Driver extends DriverAbstract
 {
-    /**
-     * @var MongodbClient
-     */
-    public $instance;
-
     /**
      * Driver constructor.
      * @param array $config
@@ -220,4 +215,4 @@ public function getStats()
 
         return $stats;
     }
-}
+}