Remove the unimplemented <input type="range" multiple>

This was added in 1efac39 as a result
of https://www.w3.org/Bugs/Public/show_bug.cgi?id=13154. Despite
definite web developer interest and some implementer support at the
time, implementation progress or commitment has not materialized in the
intervening 2.5 years, and it should be removed.

Fixes #1520.