tidy: Remove obsolete Depurate and Balancer drivers

tidy: Remove obsolete Depurate and Balancer drivers

The Html5Depurate driver was intended to be used with an external Java
service, but it never gained traction due to deployment concerns.

The Html5Internal (Balancer) driver was originally intended for use with
the balanced templates proposal and could also handle tidying. But it was
tightly coupled to MediaWiki, so part of it was used as the basis of the
RemexHtml library. Remex most likely can also implement the balanced
templates proposal, so there isn't any reason to keep the Balancer code
around anymore,

Change-Id: I8542d69e9cdbf0e2fb7ebbb919933a64c1b8c293