Proposal: drop support for XML v2 statistics

[dswarbrick]

The XML v2 stats API has been deprecated since BIND 9.10.0:

The version 3 XML schema for the statistics channel, including new statistics and a flattened XML tree for faster parsing, is no longer optional. The version 2 XML schema is now deprecated.

Additionally, BIND 9.9.x was EOL'd in July 2018.

I propose that we drop support for XML v2 stats, which would simplify some long overdue cleanup of the core of bind_exporter, and decoupling some of the overly tightly coupled XML code.

@SuperQ WDYT?

[SuperQ]

Seems reasonable to me.