claws-mail, jitsi: remove myself (khumba) from maintainers

pkgs/applications/networking/instant-messengers/jitsi/default.nix

···
     description = "Open Source Video Calls and Chat";
     license = licenses.lgpl21Plus;
     platforms = platforms.linux;
-    maintainers = with maintainers; [ khumba ndowens ];
+    maintainers = with maintainers; [ ndowens ];
   };
 }

pkgs/applications/networking/mailreaders/claws-mail/default.nix

···
     homepage = http://www.claws-mail.org/;
     license = licenses.gpl3;
     platforms = platforms.linux;
-    maintainers = with maintainers; [ khumba fpletz globin ];
+    maintainers = with maintainers; [ fpletz globin ];
   };
 }