Drop debconf support

The current logic uses debconf as a registry which is a bug
since it always gives priority what's stored there making
it impossible to update unmodified conf files.

This makes the flash-kernel-installer udeb pretty useless
but we're not using d-i anyway.
3 jobs for apparmor in 4 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #236760
librem5
builtin_tests

00:01:08

 
  Package
passed #236761
librem5:arm64
package:deb-debian-buster:arm64

00:02:07

 
  Test Package
passed #236762
librem5:arm64
lintian-debian-buster-package:arm64

00:01:42