Remove ImageVolumes from database

Before Libpod supported named volumes, we approximated image
volumes by bind-mounting in per-container temporary directories.
This was handled by Libpod, and had a corresponding database
entry to enable/disable it.

However, when we enabled named volumes, we completely rewrote the
old implementation; none of the old bind mount implementation
still exists, save one flag in the database. With nothing
remaining to use it, it has no further purpose.

Signed-off-by: Matthew Heon <mheon@redhat.com>

libpod/util_test.go

@@ -1,8 +1,9 @@
 package libpod
 
 import (
-	"github.com/stretchr/testify/assert"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestRemoveScientificNotationFromFloat(t *testing.T) {