Intent to unship: SVGSVGElement.{pixel, screenPixel}UnitToMillimeter{X, Y}

[Cameron McCormack]

In https://bugzilla.mozilla.org/show_bug.cgi?id=1133172 I plan to remove our implementations of these four IDL attributes on SVGSVGElement:

pixelUnitToMillimeterX
pixelUnitToMillimeterY
screenPixelUnitToMillimeterX
screenPixelUnitToMillimeterY

These were originally in SVG 1, but they never had much use, were never implemented in a way that was useful, and so were removed from SVG 2. Chrome removed their implementation a couple of years ago, where they found they had "virtually zero usage".

Barring objections I will land the removal in 48 hours.