[new plugin] remove-xlink-ns

[TrySound]

Ref https://github.com/svg/svgo/pull/1508

The new plugin removes xmlns:xlink from

xlink namespace is obsolete in SVG 2. Href attribute is recommended replacement to xlink:href.

This plugin will be enabled by default in SVGO 3.

See https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/href

cc @devongovett