Remove unneeded functions.

• Remove the stubs for pgpNewDig, pgpFreeDig, pgpDigGetParams, and pgpPrtPkts. They will be removed in rpm 4.19, which is the earliest released version of rpm that could use the Sequoia backend.

  • See https://github.com/rpm-software-management/rpm/pull/1992

  • See https://github.com/rpm-software-management/rpm/commit/87c4eee816a4ddd4291cf1211c0290f3a731be73