Documentation: Remove mboxctl.md

The merge of phosphor-mboxd and mboxbridge blew away the content. No-one
has complained, so remove the broken file.

Change-Id: Ic52bb4c3946b03485197efce1f6c6ff7ef714c1c
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>

Documentation/mboxctl.md

-Please reference the canonical mbox protocol documentation:
-
-    https://github.com/openbmc/mboxbridge/blob/master/Documentation/mboxctl.md