volume: store: store.go: remove debug message

When driver is `local` an empty string is given to the debug message.
This patch removes the debug statement.

```
Dec 20 19:07:01 localhost.localdomain docker[19734]:
time="2015-12-20T19:07:01.872021857+01:00" level=debug msg="Registering
new volume reference: driver , name
c2291b964b4d7b1b51ec51d2ccfe2544f83fd23404709225a43743c5faadad55"
```

Signed-off-by: Antonio Murdaca <runcom@redhat.com>