ctr: Unify the delete subcommand alias

This commit unifies the following sub commands alias for
deleting/removing.
- containers
- tasks
- contents
- leases
- images
- snapshots

Signed-off-by: Ning Li <lining2020x@163.com>

cmd/ctr/commands/images/images.go

@@ -311,8 +311,8 @@ var checkCommand = cli.Command{
 }
 
 var removeCommand = cli.Command{
-	Name:        "remove",
-	Aliases:     []string{"rm"},
+	Name:        "delete",
+	Aliases:     []string{"del", "remove", "rm"},
 	Usage:       "remove one or more images by reference",
 	ArgsUsage:   "[flags] <ref> [<ref>, ...]",
 	Description: "remove one or more images by reference",