Revert "net: sockets: Check eventfd errors better"

This reverts commit b56297bf
because it is causing problems when using native_sim and not
having CONFIG_POSIX_API set.

The proper fix is to set CONFIG_POSIX_API when using eventfd
and native_sim so that picolibc is used instead of host libc.

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