gimbal: fix unregister message subscription #3628

Job	Run time
coverage (ubuntu-20.04)	4m 52s
ubuntu-20.04 (mavsdk, hunter)	3m 56s
Ubuntu/Debian packaging (Ubuntu 20.04)	5m 45s
alpine 3.19.0 (musl)	13m 7s
linux-armv6	5m 4s
linux-armv6-musl	12m 31s
ubuntu-20.04 (package, non-mavsdk_server)	6m 34s
linux-armv7	5m 2s
ubuntu-20.04 (non-mavsdk_server, non-superbuild)	5m 14s
linux-armv7l-musl	12m 21s
linux-arm64	5m 8s
ubuntu-22.04 (package, non-mavsdk_server)	5m 37s
linux-arm64-musl	12m 47s
ubuntu-22.04 (style and proto check)	1m 24s
ubuntu-20.04 (mavsdk_server, superbuild)	12m 52s
android-arm	5m 34s
ubuntu-22.04 (mavsdk_server, superbuild)	10m 18s
android-arm64	5m 33s
ios	7m 48s
android-x86	5m 20s
ios_simulator	7m 5s
macOS	22m 31s
android-x86_64	5m 33s
macOS-framework	15m 22s
Windows	12m 18s
iOS XCFramework	1m 7s
	3h 30m 43s

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