Remove support for <math> elements in default template engine.

[theengineear]

This is seldom used and when used, typically doesn’t require interpolation. There is a simple workaround to instantiate math elements via a template’s setHTMLUnsafe method and then providing the .content document fragment to the template engine to render.

The specification for math differs from that of html. As such, supporting math adds complexity and reduces performance for a non-priority feature set.