mailman: Don't store bad messages in qfiles

In version 2.1.12, a feature was added which stores unparseable messages in
a bad directory if it couldn't be actioned. With the scale of messages sent to
lists.wikimedia.org, this causes a fairly large directory to be created which
we or mailman never uses. Since most messages are a result of client-related
encoding issues (sampled on fermium), let's not store these at all.

Bug:T110131
Change-Id: If1d5fdec1632d87f86786acc506227fad655bb94