Remove librsvg2-dev from Build-Depends

Fuzzel bundles nanosvg as an alternative SVG backend since 1.6.4,
and switched to nanosvg as the default SVG backend in 1.8.0.

librsvg is still supported as an alternative SVG backend, but since
nanosvg is the default and likely receives wider testing by fuzzel
maintainers and users, we should stick to the default for Debian.