[mdns-mdnssd] ensure allocated `DnssdHostRegistration` is freed #4708

Job	Run time
buildx (latest, ubuntu:bionic, linux/amd64,linux/arm/v7,linux/arm64, yes)	41m 31s
buildx (focal, ubuntu:focal, linux/amd64,linux/arm64, yes)	29m 59s
buildx (jammy, ubuntu:jammy, linux/amd64,linux/arm64, yes)	23m 16s
buildx (reference-device, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg BORDER_ROUTIN...	34m 15s
buildx (test, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEVICE=1 --build-...	58m 46s
docker-check (UART)	5m 21s
docker-check (SPI)	5m 6s
	3h 18m 14s

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