archive: replace tarName() with filepath.ToSlash()

This code was copied from github.com/moby/moby/pkg/archive;
28842d3f09,
which got later simplified in
a5aed699cf

This patch aligns the containerd implementation with those changes, and uses
filepath.ToSlash() unconditionally on all platforms, as it's a no-op on platforms
that use a forward-slash; https://github.com/golang/go/blob/go1.19/src/path/filepath/path.go#L175-L183

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

archive/tar_unix.go

@@ -34,10 +34,6 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-func tarName(p string) (string, error) {
-	return p, nil
-}
-
 func chmodTarEntry(perm os.FileMode) os.FileMode {
 	return perm
 }