[#1612] neofs-cli: Unify expiration flags

Use `expire-at` everywhere expiration epoch is expected.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

cmd/neofs-cli/modules/storagegroup/root.go

@@ -25,8 +25,6 @@ const (
 	cidFlag   = "cid"
 )
 
-const sgLifetimeFlag = "lifetime"
-
 func init() {
 	storageGroupChildCommands := []*cobra.Command{
 		sgPutCmd,