thermal: core: Drop redundant check from thermal_zone_device_update()

Since __thermal_zone_device_update() checks if tz->state is
TZ_STATE_READY and bails out immediately otherwise, it is not
necessary to check the thermal_zone_is_present() return value in
thermal_zone_device_update(). Namely, tz->state is equal to
TZ_STATE_FLAG_INIT initially and that flag is only cleared in
thermal_zone_init_complete() after adding tz to the list of thermal
zones, and thermal_zone_exit() sets TZ_STATE_FLAG_EXIT in tz->state
while removing tz from that list. Thus tz->state is not TZ_STATE_READY
when tz is not in the list and the check mentioned above is redundant.

Accordingly, drop the redundant thermal_zone_is_present() check from
thermal_zone_device_update() and drop the former altogether because it
has no more users.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/3406806.aeNJFYEL58@rafael.j.wysocki