confmodule: Remove unused `db_previous_module` command (Closes: #1040127)
As of 2023-07-01, the vestigial db_previous_module
command is
not mentioned in the debconf specifications [1] nor used by any
package in Debian unstable. It can thus be removed.
[1] https://www.debian.org/doc/packaging-manuals/debconf_specification.html
(Extracted from !13 (merged))