Remove the single-stage units converters

Remove the single-stage units converters

The stage-with/height-percentage converters had been broken by
the multiple-stages support of Clutter 0.8. They are also made
useless by the fact that Units are now floating point values.

The millimeters and typographic points converters also depended
on the default stage, but they can be reworked to use the default
DPI coming from the default Backend instead.