doc/security: remove old GPG information

Remove a GPG key that was lost in a server upgrade, and remove the text
that describes it.

Signed-off-by: Zac Dover <zac.dover@proton.me>

Author: zdover23

doc/security/index.rst

@@ -24,15 +24,6 @@ To report a vulnerability, please send email to `security@ceph.io
   disclosure date in mind, please share the same along with the
   report.
 
-Although you are not required to, you may encrypt your message using 
-the following GPG key:
-
-**6EEF26FFD4093B99: Ceph Security Team (security@ceph.io)**
-
-| **Download:** `MIT PGP Public Key Server <https://pgp.mit.edu/pks/lookup?op=vindex&search=0x6EEF26FFD4093B99>`_
-| **Fingerprint:** A527 D019 21F9 7178 C232 66C1 6EEF 26FF D409 3B99
-
-
 Supported versions
 ==================