mu4e: Remove signatures

lisp/init-mu4e.el

                                                                :to "aliced@home.example.com")))
            :vars '( ( user-mail-address	    . "aliced@home.example.com"  )
                     ( user-full-name	    . "Alice Derleth" )
-                    ( mu4e-compose-signature .
-                                             (concat
-                                              "Alice Derleth\n"
-                                              "Lauttasaari, Finland\n"))))
+                    ( mu4e-compose-signature . nil)))
          ,(make-mu4e-context
            :name "Work"
            :enter-func (lambda () (mu4e-message "Switch to the Work context"))
                            (string-match-p "^/Arkham" (mu4e-message-field msg :maildir))))
            :vars '( ( user-mail-address	     . "aderleth@miskatonic.example.com" )
                     ( user-full-name	     . "Alice Derleth" )
-                    ( mu4e-compose-signature  .
-                                              (concat
-                                               "Prof. Alice Derleth\n"
-                                               "Miskatonic University, Dept. of Occult Sciences\n"))))
+                    ( mu4e-compose-signature . nil)))
 
          ,(make-mu4e-context
            :name "Cycling"