No need to derive password-based key, simply skip rtl_digest_PBKDF2
for the gpg4libre case.

Also pass down PBKDF2 iteration count from ZipPackage, which knows
about GPG encryption, instead of just always setting it in package
stream.

We otherwise needlessly iterate session key also for gpg encrypted
storages.

Change-Id: Ic96b2193f8541bbd109795fb9c0212a0a10c7344
Reviewed-on: https://gerrit.libreoffice.org/47783Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>