worker: Controller.Add(): remove unused error-return #5259

Job	Run time
govulncheck	58s
prepare	6s
binaries (darwin/amd64)	1m 2s
binaries (darwin/arm64)	1m 2s
binaries (linux/amd64)	2m 3s
binaries (linux/arm/v7)	1m 56s
binaries (linux/arm64)	1m 55s
binaries (linux/s390x)	2m 23s
binaries (linux/ppc64le)	1m 55s
binaries (linux/riscv64)	2m 7s
binaries (windows/amd64)	1m 38s
binaries (windows/arm64)	1m 35s
test / prepare	1m 50s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 30s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	9m 1s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 28s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	8m 33s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 10s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 1s
test / run (oci-rootless-slirp4netns-detachnetns, ./client ./cmd/buildctl ./worker/containerd ./solver ....	5m 12s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	5m 24s
test / run (./..., 1, integration gateway)	4m 34s
test / run (./client, containerd, nydus, integration)	7m 42s
test / run (./client, oci, nydus, integration)	5m 39s
test / run (./..., nydus, 1, integration)	4m 57s
image	8m 53s
image (rootless)	9m 6s
release	7s
scout	0s
	1h 54m 47s

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