Remove obsolete $_SERVER['MAHARA_LIBDIR'] option

One of the oldest lines of code in the current Mahara codebase, is an undocumented feature in init.php. It checks for $_SERVER['MAHARA_LIBDIR'], and if this is present, it uses this location for $CFG->libdir instead of the default location $CFG->docroot/lib.

The $_SERVER superglobal is populated by the SAPI that's running PHP. If you're using mod_php, you can set it using the Apache directive "SetEnv MAHARA_LIBDIR /path/to/libdir". If you're executing PHP from the command-line, you can set it by exporting an environment fariable, i.e. "MAHARA_LIBDIR=/path/to/libdir; export MAHARA_LIBDIR; php htdocs/lib/cron.php"

There's really no reason to have this code in place. There is no supported Mahara hosting use-case where you would need the lib directory to be in a non-standard location. And as such, all this does is provide a potential vector for errors and attacks. So, I'm going to remove it.