code cleanup for ebpf ringbuf map implementation

These are some residual comments on #659
https://github.com/microsoft/ebpf-for-windows/pull/659#discussion_r753532010
https://github.com/microsoft/ebpf-for-windows/pull/659#discussion_r753532376
https://github.com/microsoft/ebpf-for-windows/pull/659#discussion_r753532598

They are all around the fact that ringbuf map supports only one consumer. So all the code to handle multiple async operation contexts can be removed to simplify code.

Another potential optimization would be to avoid to allocate a new `async_ioctl_completion_t` object for every async operation.

Start with the ringbuf map implementation and the three PR #659 review discussions linked in the issue. Trace the async operation context handling and confirm where the single-consumer constraint makes multiple contexts unnecessary. Done means the redundant handling is removed without changing ringbuf behavior; assess the per-operation completion allocation separately as a possible optimization.

Written by the indexing model from the issue text.