snapshots|pkg: umount without DETACH and nosync after umount

Signed-off-by: Wei Fu <fuweid89@gmail.com>

snapshots/testsuite/testsuite_unix.go

@@ -20,8 +20,6 @@ package testsuite
 
 import (
 	"syscall"
-
-	"golang.org/x/sys/unix"
 )
 
 func clearMask() func() {
@@ -30,7 +28,3 @@ func clearMask() func() {
 		syscall.Umask(oldumask)
 	}
 }
-
-func sync() {
-	unix.Sync()
-}