sys: move ForceRemoveAll to integration/client

ForceRemoveAll was only used in tests/fuzzing, but added hcsshim as dependency
for the sys package. Moving this to integration/client makes the "sys" package
slightly more lightweight, and may help simplifying dependency-management.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

integration/client/container_fuzzer.go

@@ -33,7 +33,6 @@ import (
 	fuzz "github.com/AdaLogics/go-fuzz-headers"
 	"github.com/containerd/containerd"
 	"github.com/containerd/containerd/oci"
-	"github.com/containerd/containerd/sys"
 	exec "golang.org/x/sys/execabs"
 )
 
@@ -125,7 +124,7 @@ func tearDown() error {
 			return err
 		}
 	}
-	if err := sys.ForceRemoveAll(defaultRoot); err != nil {
+	if err := forceRemoveAll(defaultRoot); err != nil {
 		return err
 	}