ci: rm "skip on CentOS 7" kludges

We no longer test on CentOS 7.

Remove the internal/testutil package as it has no other uses.

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

internal/testutil/testutil.go

@@ -1,30 +0,0 @@
-package testutil
-
-import (
-	"os/exec"
-	"strconv"
-	"sync"
-	"testing"
-)
-
-var (
-	centosVer     string
-	centosVerOnce sync.Once
-)
-
-func centosVersion() string {
-	centosVerOnce.Do(func() {
-		ver, _ := exec.Command("rpm", "-q", "--qf", "%{version}", "centos-release").CombinedOutput()
-		centosVer = string(ver)
-	})
-	return centosVer
-}
-
-func SkipOnCentOS(t *testing.T, reason string, versions ...int) {
-	t.Helper()
-	for _, v := range versions {
-		if vstr := strconv.Itoa(v); centosVersion() == vstr {
-			t.Skip(reason + " on CentOS " + vstr)
-		}
-	}
-}