Put the PGP foobar signature generation out of its misery

Put the PGP foobar signature generation out of its misery
- the last freeware PGP version (6.5.8) is from year 2000 and doesn't
  come close to compiling on modern distros, commercial versions we're
  not interested in
- "PGP" signatures in rpm mean RSA, gpg can handle that just fine since
  forever
- the code's been unused for years, unlikely to be functional anyway...