Copter: Use AP_Mount::clear_roi_target() to remove ROI #32825

Job	Run time
build (stm32f7, chibios, 10)	6m 4s
build (stm32h7, chibios, 10)	7m 9s
build (fmuv2-plane, chibios, 10)	2m 43s
build (periph-build, chibios, 10)	6m 50s
build (iofirmware, chibios, 10)	1m 45s
build (CubeOrange-bootloader, chibios, 10)	1m 30s
build (fmuv3-bootloader, chibios, 10)	59s
build (revo-bootloader, chibios, 10)	1m 2s
build (stm32h7-debug, chibios, 10)	3m 56s
build (fmuv3, chibios, 10)	12m 43s
build (revo-mini, chibios, 10)	5m 38s
build (MatekF405-Wing, chibios, 10)	7m 12s
build (CubeOrange-ODID, chibios, 10)	5m 15s
build (CubeRedPrimary-bootloader, chibios, 10)	1m 49s
build (configure-all, chibios, 10)	10m 4s
build (build-options-defaults-test, chibios, 10)	9m 36s
build (signing, chibios, 10)	5m 11s
	1h 29m 26s

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