thermal: int3403: remove unused struct 'int3403_performance_state'

'int3403_performance_state' has never been used since the original
commit 4384b8fe ("Thermal: introduce int3403 thermal driver").

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>