[Reminder]Decommissoned keyservers

[CHENG Gao]

In lisp/epa-ks.el, several keyservers were decommissioned, and guess
shoule be removed:

keys.gnupg.net
pool.sks-keyservers.net
zimmermann.mayfirst.org

,----
| (defcustom epa-keyserver "pgp.mit.edu"
|   "Domain of keyserver.
| 
| This is used by `epa-search-keys', for looking up public keys."
|   :type '(choice :tag "Keyserver"
|                  (repeat :tag "Random pool"
|                          (string :tag "Keyserver address"))
|                  (const "keyring.debian.org")
|                  (const "keys.gnupg.net")
|                  (const "keyserver.ubuntu.com")
|                  (const "pgp.mit.edu")
|                  (const "pool.sks-keyservers.net")
|                  (const "zimmermann.mayfirst.org")
|                  (string :tag "Custom keyserver"))
|   :version "28.1")
`----