Remove instructions on publicly certifying signing key (ref: #17544)

wiki/src/install/expert/usb.mdwn

@@ -81,20 +81,7 @@ Trust|install/download#openpgp]].
 
 1. Certify the Tails signing key with your own key:
 
-   a. To make a non-exportable certification that will never be shared
-      with others:
-
-          gpg --lsign-key A490D0F4D311A4153E2BB7CADBB802B258ACD84F
-
-   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.
+       gpg --lsign-key A490D0F4D311A4153E2BB7CADBB802B258ACD84F
 
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