STM32H747_System: remove unused variable #1301

Job	Run time
compile-examples (arduino:mbed:nano33ble, nano33ble)	6m 53s
compile-examples (arduino:mbed:envie_m7, envie_m7)	27m 0s
compile-examples (arduino:mbed:envie_m7:target_core=cm4, envie_m7_core_cm4)	5m 40s
compile-examples (arduino:mbed:nanorp2040connect, nanorp2040connect)	12m 4s
compile-examples (arduino:mbed:nicla_sense, nicla_sense)	11m 22s
compile-examples (arduino:mbed:nicla_vision, nicla_vision)	14m 36s
compile-examples (arduino:mbed:opta, opta)	17m 2s
compile-examples (arduino:mbed:giga, giga)	26m 31s
compile-examples (arduino:mbed:nicla_voice, nicla_voice)	5m 0s
compile-examples (arduino:mbed:edge_control, edge_control)	5m 20s
	2h 11m 28s

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