Remove redundant imports

```
error: the item `Ordering` is imported redundantly
  --> src/main.rs:12:5
   |
12 | use core::sync::atomic::Ordering;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
15 | use portable_atomic::*;
   |     ------------------ the item `Ordering` is already imported here
   |
   = note: `-D unused-imports` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unused_imports)]`
```

tests/avr/src/main.rs

@@ -10,8 +10,6 @@
 #[path = "../../api-test/src/helper.rs"]
 mod helper;
 
-use core::sync::atomic::Ordering;
-
 use portable_atomic::*;
 
 #[avr_device::entry]

tests/gba/src/main.rs

@@ -9,8 +9,6 @@
 #[path = "../../api-test/src/helper.rs"]
 mod helper;
 
-use core::sync::atomic::Ordering;
-
 use gba::prelude::*;
 use portable_atomic::*;
 

tests/no-std-qemu/src/main.rs

@@ -8,8 +8,6 @@
 #[path = "../../api-test/src/helper.rs"]
 mod helper;
 
-use core::sync::atomic::Ordering;
-
 use portable_atomic::*;
 use semihosting::{print, println};
 

tests/xtensa/src/main.rs

@@ -9,8 +9,6 @@
 #[path = "../../api-test/src/helper.rs"]
 mod helper;
 
-use core::sync::atomic::Ordering;
-
 use portable_atomic::*;
 
 use esp32_hal as _;