Demote this FontFactory warning to info

In most installations this causes two warnings like

** (org.inkscape.Inkscape:2243966): WARNING **: 14:03:53.037: Fonts dir '/usr/share/inkscape/fonts' does not exist and will be ignored.
** (org.inkscape.Inkscape:2243966): WARNING **: 14:03:53.037: Fonts dir '/home/mattia/.config/inkscape/fonts' does not exist and will be ignored.

That are of no interest whatsoever to the user.
If users needs to be aware of these extra directories to place their
fonts into, they should instead be documented better.

Signed-off-by: Mattia Rizzolo <mattia@mapreri.org>