mxser: inline mxser_board_remove into mxser_remove

The only user of mxser_board_remove is mxser_remove. Move there those
few lines.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-45-jslaby@suse.cz


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