RC_Channel: remove unused method get_channel_pos #34442

Job	Run time
build (chibios, Durandal)	20m 52s
build (chibios, MatekF405)	18m 5s
build (chibios, KakuteF7)	16m 22s
build (chibios, MatekH743-bdshot)	20m 48s
build (chibios, Pixhawk1-1M)	18m 37s
build (chibios, MatekF405-CAN)	2m 15s
build (chibios, DrotekP3Pro)	1m 39s
build (chibios, Hitec-Airspeed)	2m 57s
build (chibios, f103-GPS)	3m 0s
build (disco, armhf)	17m 12s
	2h 1m 47s

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