unix: remove outdated comment in TestUname, omit string conversion

Follow the review comments by Brad in CL 79918.

Change-Id: Iedb27aa9582502a90231778f5032a2e0109a4621
Reviewed-on: https://go-review.googlesource.com/80015
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_linux_test.go

@@ -191,8 +191,7 @@ func TestUname(t *testing.T) {
 		t.Fatalf("Uname: %v", err)
 	}
 
-	// conversion from []byte to string, golang.org/issue/20753
-	t.Logf("OS: %s/%s %s", string(utsname.Sysname[:]), string(utsname.Machine[:]), string(utsname.Release[:]))
+	t.Logf("OS: %s/%s %s", utsname.Sysname[:], utsname.Machine[:], utsname.Release[:])
 }
 
 func TestFstatat(t *testing.T) {