worker: Controller.Add(): remove unused error-return #4671

Job	Run time
prepare	1s
test / prepare	1m 51s
test / run (containerd, ./frontend/dockerfile, integration)	4m 55s
test / run (containerd, ./frontend/dockerfile, dockerfile)	6m 57s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 38s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	6m 52s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	4m 39s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	6m 39s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 16s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	7m 2s
test / run (oci, ./frontend/dockerfile, integration)	4m 18s
test / run (oci, ./frontend/dockerfile, dockerfile)	6m 5s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 17s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	5m 28s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 27s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	5m 49s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 8s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	5m 49s
image (mainline)	5m 8s
image (labs)	4m 56s
scout	0s
release	0s
	1h 40m 15s

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