debian/control: No longer depend on debconf (Closes #1006492)

In 2007 cdebconf has been "temporarily" made to depend on debconf

to avoid anyone breaking their systems by removing debconf, which dependencies now allow.

(See commit b4dfa070)

Now, 15 years and various Debian releases later, all packages in the base system depend on some variation of debconf | debconf-2.0.

This hard-dependence can now be removed.