config: remove unnecessay env XDEBUG_MODE

It does not work. See #6274 (comment)

Author: kenjis

phpunit.xml.dist

@@ -70,7 +70,6 @@
     </extensions>
 
     <php>
-        <env name="XDEBUG_MODE" value="coverage"/>
         <server name="app.baseURL" value="http://example.com/"/>
         <!-- Directory containing phpunit.xml -->
         <const name="HOMEPATH" value="./"/>