There are no more users of get_thermal_instance(), so drop it.

While at it, replace get_instance() returning a pointer to struct
thermal_instance with thermal_instance_present() returning a bool
which is more straightforward.

No functional impact.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://patch.msgid.link/2014591.usQuhbGJ8B@rjwysocki.net
[ rjw: Dropped get_thermal_instance() documentation ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>