Remove "Resolution" drop-downs from metrics website

We added drop-down menus for the graph image resolution almost two years ago. The idea was to use a higher bitmap resolution for reports or slides. I think this was a feature request by phobos back then.

With legacy/trac#7050 (moved) being implemented, we now have PDF and SVG graph outputs that can be used for reports or slides even better.

Can we take out resolution drop-downs now?

This would make the website a bit clearer by removing an option that the average user does not need, and it would make the code less complex and easier to maintain.