igloo: remove the server side of the former graph renderer

The commits before this one moved every page onto the engine, so the
code that built the former renderer's HTML has no callers left. Of its
1582 lines, _html_render keeps the 137 that other callers still reach:
the export request model, the error box, the hover popup and the
destination choices.

Its two endpoints go with it, ajax_render_graph and
ajax_graph_values_at_time, plus the artwork helper that only fed the
latter. The tests drop the unit coverage of the removed internals and
the E2E assertions that checked the former DOM beside the engine's.

The image pipeline is untouched: PDF reports, notification PNGs and
graph export still render through _artwork and _graph_images.

CMK-37237

Change-Id: I16e8e79d06c53d1c1ba529e15a42c49e8c59e372