remmina: remove webkitgtk dependency

It was used by "survey" module which was removed in revision
d97920eb9b32dbe17b85f8eff12928d26c19fd3d of their repository
on 2016-12-13.

Meanwhile, WebKit plugin is a separate package
https://aur.archlinux.org/packages/remmina-plugin-webkit/
not packaged in nixpkgs.