Remove encryption option for volumes

Change Idb6a923f (https://review.openstack.org/#/c/21264/) added an
encryption choice field for volumes.  However, this choice field uses
volume metadata rather than volume type to denote an encrypted volume.
This option is removed in this patch, since the existing volume type
field should be used instead.

Change-Id: I730b4ffc26f391065943cb0d135ceac113cb6754
Implements: blueprint volume-encryption-field
Merge "Remove encryption option for volumes"