Remove SVG1.1 kerning property

This SVG1.1[1] property has been removed from SVG2[2] and can be removed.

[1] http://www.w3.org/TR/SVG11/single-page.html#text-KerningProperty
[2] https://svgwg.org/svg2-draft/single-page.html

Review URL: https://codereview.chromium.org/234453002

git-svn-id: svn://svn.chromium.org/blink/trunk@171393 bbb929c8-8fbe-4397-9dbb-9b2b20218538