Staging: hv: remove vmbus_loglevel as it is not used at all anymore

As there is no user of this variable, it's time to delete it.  For
dynamic debugging of the hyperv code, use the standard dynamic debug
kernel interface.

Cc: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>