Remove workaround for old rollup bug

modules/behavior/breathe.js

@@ -1,5 +1,4 @@
 import * as d3 from 'd3';
-import { transition as d3transition } from 'd3';
 import _ from 'lodash';
 
 
@@ -14,13 +13,6 @@ export function behaviorBreathe() {
         timer;
 
 
-    // This is a workaround for a bug in rollup v0.36
-    // https://github.com/rollup/rollup/issues/984
-    // The symbol for this default export is not used anywhere, but it
-    // needs to be called in order to be included in the bundle.
-    var t = d3transition();
-
-
     function ratchetyInterpolator(a, b, steps, units) {
         a = parseFloat(a);
         b = parseFloat(b);