x11-fonts/sharefonts: Do not package fonts.dir

It is generated by USES=font post-install. Packaging it just
leads to checksum errors:

Checking for packages with mismatched checksums:
sharefonts-0.10_6: /usr/local/share/fonts/sharefonts/fonts.dir

PR: 224407
Reported by: Marco Beishuizen <mbeis@xs4all.nl>