libct/cgroupv1/getCgroupMountsHelper: minor nit

It is easy to just use TrimPrefix which does nothing in case the prefix
does not exist.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/cgroupv1/utils.go

@@ -151,9 +151,7 @@ func getCgroupMountsHelper(ss map[string]bool, mi io.Reader, all bool) ([]Mount,
 				continue
 			}
 			ss[opt] = true
-			if strings.HasPrefix(opt, CgroupNamePrefix) {
-				opt = opt[len(CgroupNamePrefix):]
-			}
+			opt = strings.TrimPrefix(opt, CgroupNamePrefix)
 			m.Subsystems = append(m.Subsystems, opt)
 			numFound++
 		}