Serial: remove intermediate_buffer leftover #1207

Job	Run time
compile-examples (arduino:mbed:nano33ble, nano33ble)	12m 21s
compile-examples (arduino:mbed:envie_m7, envie_m7)	16m 38s
compile-examples (arduino:mbed:envie_m7:target_core=cm4, envie_m7_core_cm4)	4m 19s
compile-examples (arduino:mbed:nanorp2040connect, nanorp2040connect)	21m 37s
compile-examples (arduino:mbed:nicla_sense, nicla_sense)	22m 56s
compile-examples (arduino:mbed:nicla_vision, nicla_vision)	19m 8s
compile-examples (arduino:mbed:opta, opta)	23m 48s
compile-examples (arduino:mbed:giga, giga)	38m 54s
compile-examples (arduino:mbed:nicla_voice, nicla_voice)	8m 0s
compile-examples (arduino:mbed:edge_control, edge_control)	8m 34s
	2h 56m 15s

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