bboltcachestorage: delete links outside of cursor #5959

Job	Run time
sandbox-build (linux/amd64)	6m 48s
build (windows/amd64)	1m 54s
sandbox-build (linux/arm64)	4m 32s
build (freebsd/amd64)	1m 58s
test-freebsd-amd64	2m 56s
test-windows-amd64 (containerd, ./client)	7m 38s
test-windows-amd64 (containerd, ./cmd/buildctl)	5m 8s
test-windows-amd64 (containerd, ./worker/containerd)	3m 19s
test-windows-amd64 (containerd, ./solver)	3m 20s
test-windows-amd64 (containerd, ./frontend)	3m 18s
test-windows-amd64 (containerd, ./frontend/dockerfile)	42m 29s
test-windows-amd64 (./..., 1)	4m 50s
	1h 28m 10s

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