d/control: Remove outdated debconf version constraint (Closes: #1096117)

Gioele Barabuccirequested to merge
gioele/firebird4.0:debconf-dep into main

debconf 1.4.69 is older than Debian 4 "etch" (released in 2007), thus no longer needed.

An appropriate dependency will be added by dh_installdebconf.

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