buflist, irc, xfer: do not translate "cleartext" in buffer local variable "tls_version" (issue #1622)

The text "cleartext" is translated for display only: directly with gettext in
IRC plugin and with "${translate:${tls_version}}" in the buflist evaluated
format.

src/plugins/irc/irc-server.c

@@ -1139,7 +1139,7 @@ irc_server_set_tls_version (struct t_irc_server *server)
         else
         {
             weechat_buffer_set (server->buffer, "localvar_set_tls_version",
-                                _("cleartext"));
+                                "cleartext");
         }
     }
     else