vold hasn't use the generic "block_device" label since
commit 273d7ea4 (Sept 2014), and
the auditallow statement in vold hasn't triggered since that time.

Remove the rule which allows vold access to the generic block_device
label, and remove the vold exception.

Thanks to jorgelo for reminding me about this.

Change-Id: Idd6cdc20f5be9a40c5c8f6d43bbf902a475ba1c9