Remove unused system.Execv #6051

Job	Run time
cross-i386	1m 22s
cross-i386 (runc_nodmz)	1m 39s
test (ubuntu-20.04, 1.20.x, rootless, -race)	6m 34s
test (ubuntu-20.04, 1.20.x, rootless)	6m 8s
test (ubuntu-20.04, 1.20.x, -race)	7m 15s
test (ubuntu-20.04, 1.20.x)	7m 37s
test (ubuntu-20.04, 1.21.x, rootless, -race)	6m 15s
test (ubuntu-20.04, 1.21.x, rootless)	6m 10s
test (ubuntu-20.04, 1.21.x, -race)	7m 17s
test (ubuntu-20.04, 1.21.x)	6m 19s
test (ubuntu-20.04, 1.21.x, criu-dev)	6m 56s
test (ubuntu-22.04, 1.20.x, rootless, -race)	10m 10s
test (ubuntu-22.04, 1.20.x, rootless)	9m 53s
test (ubuntu-22.04, 1.20.x, -race)	8m 21s
test (ubuntu-22.04, 1.20.x)	7m 46s
test (ubuntu-22.04, 1.21.x, rootless, -race)	10m 27s
test (ubuntu-22.04, 1.21.x, rootless)	9m 47s
test (ubuntu-22.04, 1.21.x, -race)	8m 37s
test (ubuntu-22.04, 1.21.x)	7m 49s
test (ubuntu-22.04, 1.21.x, runc_nodmz)	7m 49s
test (ubuntu-22.04, 1.21.x, criu-dev)	8m 23s
test (actuated-arm64-6cpu-8gb, 1.21.x, rootless)	10m 45s
test (actuated-arm64-6cpu-8gb, 1.21.x)	6m 47s
	2h 50m 6s

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