Removes UIDevice uniqueIdentifier call

As of Xcode 6, uniqueIdentifier is not available to 3rd party vendors.

We thought this might be causing this output in iOS 8 GM, but it is not.

```
MobileGestaltHelper[909] <Error>: libMobileGestalt MobileGestalt.c:273: server_access_check denied access to question UniqueDeviceID for pid 796 
ScriptAgent[796] <Error>: libMobileGestalt MobileGestaltSupport.m:170: pid 796 (ScriptAgent) does not have sandbox access for re6Zb+zwFKJNlkQTUeT+/w and IS NOT appropriately entitled
ScriptAgent[703] <Error>: libMobileGestalt MobileGestalt.c:534: no access to UniqueDeviceID (see <rdar://problem/11744455>)
```

### On Device Tests

Locally against iOS 6, 7, and 8 GM.