Revert "drivers: serial: ns16550: Add support for Async APIs"

This reverts commit 2d03aaf9.

The async API for NS16550 is incomplete. We are near the next
release so it is better to revert it for now, and a proper
correct implementation can be done before next-next release.

Relates to #57103

Signed-off-by: Daniel Leung <daniel.leung@intel.com>