libpod: switch to golang native error wrapping

We now use the golang error wrapping format specifier `%w` instead of
the deprecated github.com/pkg/errors package.

[NO NEW TESTS NEEDED]

Signed-off-by: Sascha Grunert <sgrunert@redhat.com>

libpod/shutdown/handler.go

@@ -1,18 +1,18 @@
 package shutdown
 
 import (
+	"errors"
 	"os"
 	"os/signal"
 	"sync"
 	"syscall"
 	"time"
 
-	"github.com/pkg/errors"
 	logrusImport "github.com/sirupsen/logrus"
 )
 
 var (
-	ErrHandlerExists error = errors.New("handler with given name already exists")
+	ErrHandlerExists = errors.New("handler with given name already exists")
 )
 
 var (