Remove a redundant import to silence a warning

```
error: the item `atomic` is imported redundantly
  --> src/backend/mod.rs:35:5
   |
35 | use atomic;
   |     ^^^^^^
   |
  ::: src/lib.rs:6:1
   |
6  | extern crate atomic;
   | -------------------- the item `atomic` is already imported here
```

Author: Pehrsons

src/backend/mod.rs

@@ -32,7 +32,6 @@ use self::device_property::*;
 use self::mixer::*;
 use self::resampler::*;
 use self::utils::*;
-use atomic;
 use backend::ringbuf::RingBuffer;
 use cubeb_backend::{
     ffi, ChannelLayout, Context, ContextOps, DeviceCollectionRef, DeviceId, DeviceRef, DeviceType,