daemon/graphdriver/copy: rm dead code

Here, err is never non-nil as it was checked earlier.

Fixes the following linter warning:

> daemon/graphdriver/copy/copy.go:136:10: nilness: impossible condition: nil != nil (govet)
>		if err != nil {
>		       ^

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

daemon/graphdriver/copy/copy.go

@@ -133,9 +133,6 @@ func DirCopy(srcDir, dstDir string, copyMode Mode, copyXattrs bool) error {
 		}
 
 		dstPath := filepath.Join(dstDir, relPath)
-		if err != nil {
-			return err
-		}
 
 		stat, ok := f.Sys().(*syscall.Stat_t)
 		if !ok {