There is not a valid reason to attempt setting the monitor channel
while in managed mode. Since this code path only deals with this mode,
remove the code block.

Johannes: I'll note that the comment indicated it was for backward
compatibility, but the code wasn't functional since switching the
monitor channel isn't supported (any more?) when in managed mode, as
that mode owns the channel configuration. Additionally, since monitor
can't be done on a managed mode interface, this would only have had
any effect to start with if a separate monitor interface is present,
in which case it's better to change the channel through that anyway,
if even possible.
Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>