Revert "aio: add aio support for iov_iter arguments"

This reverts commit 52bb8ea2e685ad6ad414a77e0a2ebbe7a38413c8.
Revert "aio: add aio_kernel_() interface"

This reverts commit f308c9d7bfae11e3588492d5a51b9a3a59c79c91.
Revert "loop: use aio to perform io on the underlying file"

This reverts commit 8e09ca4213515592450aaeb234a1219bb8e8c360.
Revert "aio: convert the ioctx list to radix tree"

This reverts commit 603655aca24de924ffc96045fdee3e47a5e9d8aa.