Remove unused private member to get rid of compiler warning in Qurt build #43965

Job	Run time
build (stm32f7, chibios, 10)	3m 40s
build (stm32h7, chibios, 10)	3m 33s
build (fmuv2-plane, chibios, 10)	1m 49s
build (periph-build, chibios, 10)	3m 49s
build (iofirmware, chibios, 10)	1m 21s
build (CubeOrange-bootloader, chibios, 10)	50s
build (fmuv3-bootloader, chibios, 10)	54s
build (revo-bootloader, chibios, 10)	49s
build (stm32h7-debug, chibios, 10)	2m 41s
build (fmuv3, chibios, 10)	5m 53s
build (revo-mini, chibios, 10)	4m 34s
build (MatekF405-Wing, chibios, 10)	4m 46s
build (CubeOrange-ODID, chibios, 10)	2m 51s
build (CubeRedPrimary-bootloader, chibios, 10)	1m 0s
build (configure-all, chibios, 10)	11m 16s
build (build-options-defaults-test, chibios, 10)	7m 26s
build (signing, chibios, 10)	2m 57s
build (CubeOrange-PPP, chibios, 10)	2m 0s
build (CubeRed-EKF2, chibios, 10)	2m 1s
build (SOHW, chibios, 10)	3m 22s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 35s
build (new-check, chibios, 10)	4m 18s
	1h 13m 25s

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