ContainerRegistry: Remove backported async URLSession functions on Linux (#12)

### Motivation

As of Swift 6.0, Foundation includes these async URLSession functions on Linux so backports are no longer necessary.

### Modifications

Remove the extension which backported async `upload()` and `data()` to `URLSession` on Linux.
 
### Result

No functional change.

### Test Plan

Unit and integration tests continue to pass.

Author: euanh