libimage/manifests: fix a gosimple linter warning

> libimage/manifests/manifests.go:408:3: S1033: unnecessary guard around call to delete (gosimple)
> 		if _, needToDelete := l.instances[instanceDigest]; needToDelete {
> 		^

Indeed, we can just call delete right away.

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

common/libimage/manifests/manifests.go

@@ -405,9 +405,7 @@ func (l *list) Add(ctx context.Context, sys *types.SystemContext, ref types.Imag
 func (l *list) Remove(instanceDigest digest.Digest) error {
 	err := l.List.Remove(instanceDigest)
 	if err == nil {
-		if _, needToDelete := l.instances[instanceDigest]; needToDelete {
-			delete(l.instances, instanceDigest)
-		}
+		delete(l.instances, instanceDigest)
 	}
 	return err
 }