Remove unused variable assignments

exceptions/updates/MDInvalidCurrency.php

@@ -20,8 +20,7 @@ final class MDInvalidCurrency extends MDgenericInvalidInputsException {
      */
     public function errorMessage() {
         //error message
-        $errorMsg = 'The following currency is not available: ' . $this->getMessage();
-        return $errorMsg;
+        return 'The following currency is not available: ' . $this->getMessage();
 
     }
 }