Remove ttf-liberation dependency

Description:

ttf-liberation is only a dependency because Calibre manually loads these fonts into Qt’s Font manager.

Qt is perfectly capable of finding these fonts by itself, so please make the dependency optional.

Calibre works perfectly if you just delete $pkgdir/usr/share/calibre/fonts/liberation (which contains nothing but symlinks).

The step python setup.py liberation_fonts --system-liberation_fonts ... is still necessary, otherwise Calibre downloads the fonts.