Remove extraneous g elements, simplify tick code.

Speeds up animations in Firefox, which gradually slows down and in some cases hangs before convergence on complex networks (see https://bugzilla.mozilla.org/show_bug.cgi?id=791699).
Disable animation in Firefox.

Until Firefox is fixed, non-trivial SVG animations will cause it to slow down and potentially hang. https://bugzilla.mozilla.org/show_bug.cgi?id=791699