libcontainer/cgroups/fs: remove todo since strings.Fields performs well #6578

Job	Run time
cross-i386	53s
cross-i386 (runc_nodmz)	47s
test (ubuntu-20.04, 1.22.x, rootless, -race)	6m 54s
test (ubuntu-20.04, 1.22.x, rootless)	6m 59s
test (ubuntu-20.04, 1.22.x, -race)	7m 52s
test (ubuntu-20.04, 1.22.x)	7m 23s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 12s
test (ubuntu-20.04, 1.23.x, rootless)	6m 57s
test (ubuntu-20.04, 1.23.x, -race)	8m 0s
test (ubuntu-20.04, 1.23.x)	7m 20s
test (ubuntu-20.04, 1.23.x, criu-dev)	7m 45s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 38s
test (ubuntu-24.04, 1.22.x, rootless)	9m 40s
test (ubuntu-24.04, 1.22.x, -race)	8m 8s
test (ubuntu-24.04, 1.22.x)	7m 49s
test (ubuntu-24.04, 1.23.x, rootless, -race)	9m 21s
test (ubuntu-24.04, 1.23.x, rootless)	9m 8s
test (ubuntu-24.04, 1.23.x, -race)	7m 52s
test (ubuntu-24.04, 1.23.x)	6m 34s
test (ubuntu-24.04, 1.23.x, runc_nodmz)	6m 45s
test (ubuntu-24.04, 1.23.x, criu-dev)	7m 3s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 48s
test (actuated-arm64-6cpu-8gb, 1.23.x)	8m 45s
all-done	1s
	2h 50m 34s

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