Disable liberation fonts

[David Demelier]

Hello,

I've installed editors/libreoffice, but that port requires liberation
fonts. I have nothing against that but when liberations font is
installed the fonts looks so ugly in firefox and sometime when I click
links, all the text in the page get bold for a second, it's just annoying.

I'm not used to fontconfig sadly, I just want to disable every
liberation fonts but not deinstall it (to use it if needed in libreoffice).

Regards,
David.