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 {
>		       ^
@@ -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 {
