mxser: Documentation, remove traces of callout device

Callout devices are ancient history, so remove its traces from mxser's
documentation.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-68-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>