AP_ExternalAHRS: Remove incorrect 0xFFFF default comment #37131

Job	Run time
build (stm32f7, chibios, 10)	3m 52s
build (stm32h7, chibios, 10)	3m 29s
build (fmuv2-plane, chibios, 10)	2m 9s
build (periph-build, chibios, 10)	3m 38s
build (iofirmware, chibios, 10)	1m 23s
build (CubeOrange-bootloader, chibios, 10)	56s
build (fmuv3-bootloader, chibios, 10)	48s
build (revo-bootloader, chibios, 10)	1m 12s
build (stm32h7-debug, chibios, 10)	2m 23s
build (fmuv3, chibios, 10)	5m 17s
build (revo-mini, chibios, 10)	4m 11s
build (MatekF405-Wing, chibios, 10)	4m 31s
build (CubeOrange-ODID, chibios, 10)	2m 51s
build (CubeRedPrimary-bootloader, chibios, 10)	49s
build (configure-all, chibios, 10)	9m 18s
build (build-options-defaults-test, chibios, 10)	7m 28s
build (signing, chibios, 10)	2m 51s
build (CubeOrange-PPP, chibios, 10)	2m 3s
build (SOHW, chibios, 10)	3m 17s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 19s
	1h 3m 45s

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