thermal: core: Drop dead code from monitor_thermal_zone()

thermal: core: Drop dead code from monitor_thermal_zone()

Since monitor_thermal_zone() is only called when the given thermal zone
has been enabled, as per the thermal_zone_device_is_enabled() check in
__thermal_zone_device_update(), the tz->mode check in it always
evaluates to "false" and the thermal_zone_device_set_polling()
invocation depending on it is dead code, so drop it.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/10547425.nUPlyArG6x@rjwysocki.net