sensorhandler: removed un-needed sd_bus_ref
sensorhandler: removed un-needed sd_bus_ref

sdbusplus's bus object has a change in semantics that no longer
require a 'sd_bus_ref' call.  See changes for openbmc/openbmc#1432.

Change-Id: I6e953cfbd83abec1534e75b8fbf6c7e5b711ceb3
Signed-off-by: Patrick Williams <cbb7353e6d953ef360baf960c122346276c6e320@stwcx.xyz>

openbmc/phosphor-host-ipmid,openbmc/phosphor-host-ipmid,openbmc/phosphor-host-ipmid