Change the default for DynamicDHGroups to 0

This feature can make Tor relays less identifiable by their use of the
mod_ssl DH group, but at the cost of some usability (#4721) and bridge
tracing (#6087) regressions.

We should try to turn this on by default again if we find that the
mod_ssl group is uncommon and/or we move to a different DH group size
(see #6088).  Before we can do so, we need a fix for bugs #6087 and

Resolves ticket #5598 for now.