monitor: Clean up how monitor_disas() funnels output to monitor

INIT_DISASSEMBLE_INFO() takes an fprintf()-like callback and a FILE *
to pass to it.  monitor_disas() passes monitor_fprintf() and the
current monitor cast to FILE *.  monitor_fprintf() casts it right
back, and is otherwise identical to monitor_printf().  The
type-punning is ugly.

Pass qemu_fprintf() and NULL instead.

monitor_fprintf() is now unused; delete it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190417191805.28198-16-armbru@redhat.com>
[Commit message typo corrected]