Remove too generic magic from application/x-pak

This is the Quake PAK format, and it does start with PACK, but the real signature requires parsing the file deeper, as explained on https://web.archive.org/web/20070701221700/http://mediasrv.ns.ac.yu:80/extra/fileformat/games/pak/pak.txt

References:

• https://bugs.freedesktop.org/show_bug.cgi?id=10030
• https://invent.kde.org/unmaintained/kdelibs/-/blob/KDE/3.5/kio/magic?ref_type=heads

Fixes #210 (closed)