staging: dgnc: remove crazy "management" device node

There's no need for a special character device just to get some random
information out of a single serial port driver.  So remove the
dgnc_mgmt.c file, and some structures and ioctl definitions that only it
was using.

Cc: Lidza Louina <lidza.louina@gmail.com>
Cc: Mark Hounschell <markh@compro.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>