dt-bindings: thermal: Drop db8500-thermal.txt

The binding is already defined in mfd/stericsson,db8500-prcmu.yaml and none
of 'the tripN.*' properties appear to be in use.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20251030195234.439141-1-robh@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Author: robherring