textproc/py-sphinxcontrib-svg2pdfconverter: depend on correct librsvg2 automatically

Instead of providing a port option, have USES=gnome figure out the
right librsvg2 to depend on. This avoids conflicts when installing mame
with other ports that depend on librsvg2{,-rust}.

Approved by: agh@riseup.net (maintainer)
MFH: 2023Q2
PR: 271530