MINOR: config: remove obsolete use of DEFAULT_MAXCONN at various places

MINOR: config: remove obsolete use of DEFAULT_MAXCONN at various places

This entry was still set to 2000 but never used anymore. The only places
where it appeared was as an alias to SYSTEM_MAXCONN which forces it, so
let's turn these ones to SYSTEM_MAXCONN and remove the default value for
DEFAULT_MAXCONN. SYSTEM_MAXCONN still defines the upper bound however.