Revert r44254 "Cleanup to wfGetNull(): * Add override in case /dev/null isn't accessi...

Revert r44254 "Cleanup to wfGetNull(): * Add override in case /dev/null isn't accessible (bug 11009)"
Doesn't seem like a proper fix here... At best, this'll be dumping random crap to some random file unless the user has a local copy of the /dev/null device file, which seems.... wrong. :)
For anything generating command lines, it probably won't make any difference (assuming exec() is enabled at all!) since open_basedir won't be searching through the command line (I think).
Assuming the core use case actually happens (open_basedir is set, but proc_open() is available to run tidy), a more correct fix is probably to go ahead and read in stderr and toss the results, or maybe better pass it through to PHP's stderr FD instead of opening /dev/null ourselves. Tidy has a -q option which should suppress random "hi i'm tidy version XYZ" if it's currently present.