Bug 1410796 - Drop element.isSVGElement. r=whimboo

This drops the now-superfluous element.isSVGElement.  SVG elements
are matched with element.isDOMElement because there is no special
need to distinguish them from other elements, apart from chrome elements.

MozReview-Commit-ID: 3AgEtyaAsPU

--HG--
extra : rebase_source : 7ec52aa9a7b7404eff5b6daf8ebe8259f06265da