igloo: drop the vue-graphing-enabled flag from the forecast pages

The forecast pages were the last readers of the flag, so the URL
parameter is gone and every graph page renders through the engine. That
leaves render_graphs_html without callers, and the graph popup no longer
needs a whole graph environment just to pass a debug flag along.

CMK-37237

Change-Id: Ifbbbce757fb2b9bb855048e72ebb8b514d8861f0