net: sockets: Remove async service support

As found in PR #75525, we should not modify the polled fd array
in multiple places. Because of this fix, the async version of
the socket service could start to trigger while it is being handled
by the async handler. This basically means that the async version
cannot work as intended so remove its support.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>