Expert verification instructions encourage users to certify our key publicly

Originally created by @intrigeri on #17544 (Redmine)

   b. To make an exportable certification of the Tails signing
      key and publish it on the public key servers:

          gpg --sign-key A490D0F4D311A4153E2BB7CADBB802B258ACD84F
          gpg --send-keys A490D0F4D311A4153E2BB7CADBB802B258ACD84F

      Doing so allows people who verified
      your key to verify your certification and, as a consequence, build
      more trust in the Tails signing key.

I don’t think that the level of verification documented in the preceding instructions justifies a public certification of our key.
Encouraging users to do so:

• spreads suboptimal OpenPGP key signing practices
• makes our key grow larger and larger