d/control: Remove outdated debconf version constraint (Closes: #1096119)

Gioele Barabuccirequested to merge
gioele/isc-kea:debconf-dep into debian/unstable

debconf 0.5 is older than Debian 3.0 "woody" (released in 2002), thus this version constraint is not really useful.

An appropriate dependency (compatible with cdebconf) will be added by dh_installdebconf.

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