Remove encryption code from containerd core

We are separating out the encryption code and have designed a few new
interfaces and APIs for processing content streams.  This keep the core
clean of encryption code but enables not only encryption but support of
multiple content types ( custom media types ).

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cmd/ctr/commands/commands.go

@@ -131,22 +131,6 @@ var (
 			Usage: "add a device to a container",
 		},
 	}
-	// ImageDecryptionFlags are cli flags needed when decrypting an image
-	ImageDecryptionFlags = []cli.Flag{
-		cli.StringFlag{
-			Name:  "gpg-homedir",
-			Usage: "The GPG homedir to use; by default gpg uses ~/.gnupg",
-		}, cli.StringFlag{
-			Name:  "gpg-version",
-			Usage: "The GPG version (\"v1\" or \"v2\"), default will make an educated guess",
-		}, cli.StringSliceFlag{
-			Name:  "key",
-			Usage: "A secret key's filename and an optional password separated by colon; this option may be provided multiple times",
-		}, cli.StringSliceFlag{
-			Name:  "dec-recipient",
-			Usage: "Recipient of the image; used only for PKCS7 and must be an x509 certificate",
-		},
-	}
 )
 
 // ObjectWithLabelArgs returns the first arg and a LabelArgs object