Mentors should strip GPG signatures

Mentors should strip GPG signatures from imported .dsc files (and .changes file, if we decided not to throw them away). Some DDs (harrumph, ok me) use mentors.d.n to exchange packages not ready to be uploaded.

However, these packages might contain a signature in the Debian keyring which could be uploaded by everyone to the Debian ftp master (or uploaded again to mentors - if anyone cared)

[ originally reported by @arno at https://alioth.debian.org/tracker/index.php?func=detail&aid=313686&group_id=100127&atid=413115 ]