[host] remove unused methods in RcpHost #6777

Job	Run time
buildx (latest, ubuntu:bionic, linux/amd64,linux/arm/v7,linux/arm64, yes)	30m 57s
buildx (latest, ubuntu:bionic, linux/amd64,linux/arm/v7,linux/arm64, yes)	30m 57s
buildx (focal, ubuntu:focal, linux/amd64,linux/arm64, yes)	22m 37s
buildx (focal, ubuntu:focal, linux/amd64,linux/arm64, yes)	22m 37s
buildx (jammy, ubuntu:jammy, linux/amd64,linux/arm64, yes)	20m 43s
buildx (jammy, ubuntu:jammy, linux/amd64,linux/arm64, yes)	20m 43s
buildx (reference-device, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg BORDER_ROUTIN...	7m 45s
buildx (reference-device, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg BORDER_ROUTIN...	28m 30s
buildx (test, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEVICE=1 --build-...	31m 52s
buildx (test, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEVICE=1 --build-...	31m 52s
docker-check	4m 4s
docker-check	4m 4s
	4h 16m 41s

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