Depend on libproxy1v5 instead of plugins

libproxy 0.5 was just uploaded to Unstable. It is a complete rewrite and no longer has a plugin architecture.

Therefore, it is necessary to drop the Depends on packages that no longer exist (temporarily provided as transitional packages).

It is less common for a metapackage to directly depend on a library package, but I don't see any obvious reverse-dependency of the library to depend on instead. But let's see…

GNOME and GNOME-related desktops will have libproxy because the help viewer app yelp depends on libsoup3 which depends on glib-networking which depends on libproxy. A KDE desktop probably has it, but not sure about other desktops.

Closes: https://bugs.debian.org/1064900