MEDIUM: ssl: increase default-dh-param to 2048

MEDIUM: ssl: increase default-dh-param to 2048

For 6 years now we've been seeing a warning suggesting to set dh-param
beyond 1024 if possible when it was not set. It's about time to do it
and get rid of this warning since most users seem to already use 2048.
It will remain possible to set a lower value of course, so only those
who were experiencing the warning and were relying on the default value
may notice a change (higher CPU usage). For more context, please refer
to this thread :

  https://www.mail-archive.com/haproxy@formilux.org/msg37226.html

This commit removes a big chunk of code which happened to be needed
exclusively to figure if it was required to emit a warning or not :-)