Remove very old information

wiki/src/doc/about/openpgp_keys.mdwn

@@ -29,15 +29,6 @@ Purpose
 This key has an encryption subkey. Please use it to encrypt email sent
 to the core developers encrypted mailing list: [[tails@boum.org|about/contact#tails]].
 
-### Signature
-
-This key also has the capability to sign and certify. Until Tails
-0.5 and 0.6~rc3, released images were signed by this key. This purpose
-is now deprecated: further releases will be signed by a dedicated,
-safer signing key. As of 2010 October 7th, our mailing list key
-signature only means our mailing list software checked the signed
-content was originally OpenPGP-signed by a Tails core developer.
-
 Policy
 ------