STM32H747_System: remove unused variable #1300

Job	Run time
compile-examples (arduino:mbed:nano33ble, nano33ble)	6m 57s
compile-examples (arduino:mbed:envie_m7, envie_m7)	28m 9s
compile-examples (arduino:mbed:envie_m7:target_core=cm4, envie_m7_core_cm4)	6m 56s
compile-examples (arduino:mbed:nanorp2040connect, nanorp2040connect)	11m 59s
compile-examples (arduino:mbed:nicla_sense, nicla_sense)	12m 20s
compile-examples (arduino:mbed:nicla_vision, nicla_vision)	15m 14s
compile-examples (arduino:mbed:opta, opta)	17m 16s
compile-examples (arduino:mbed:giga, giga)	26m 15s
compile-examples (arduino:mbed:nicla_voice, nicla_voice)	4m 55s
compile-examples (arduino:mbed:edge_control, edge_control)	5m 3s
	2h 15m 4s

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