igloo: drop the legacy dashboard graph branch

DashboardContent routes every graph type to the time series graph widget
in a case that precedes GRAPH_TYPES, so the legacy branch behind it was
already unreachable. Removing it strands the graph widget and hover
pages, the two figure modules they fed and the dashlet renderer.

CMK-37237

Change-Id: I2c9591d0aec3762026aae4703ea02e984115d371