httpd module: don't create documentRoot directory if it doesn't exist

It hides bugs and do you ever actually want to serve up an empty directory?
It was pretty confusing to me when it tried to write into a read-only store
path because I accidentally pointed it to the wrong store path.

nixos/modules/services/web-servers/apache-httpd/default.nix

@@ -709,13 +709,6 @@ in
             ''}
             mkdir -m 0700 -p ${mainCfg.logDir}
 
-            ${optionalString (mainCfg.documentRoot != null)
-            ''
-              # Create the document root directory if does not exists yet
-              mkdir -p ${mainCfg.documentRoot}
-            ''
-            }
-
             # Get rid of old semaphores.  These tend to accumulate across
             # server restarts, eventually preventing it from restarting
             # successfully.