SVGLangSpace interface should be removed

Remove the SVGLangSpace interface as dictated by the SVG2 specification:

https://svgwg.org/svg2-draft/types.html#InterfaceSVGElement

The xml:lang handling is handled purely through get/setAttribute. The xml:space uses the same approach, but care
has to be taken in SVGTextContentElement to deal with xml:space as we want to react to attribute value changes.

BUG=253477

Review URL: https://codereview.chromium.org/109433003

git-svn-id: svn://svn.chromium.org/blink/trunk@163723 bbb929c8-8fbe-4397-9dbb-9b2b20218538