net: sockets: Remove deperecated CONFIG_NET_SOCKETS_POSIX_NAMES

The deprecated CONFIG_NET_SOCKETS_POSIX_NAMES option is removed.
If one wishes to use POSIX API socket calls, then CONFIG_POSIX_API
option needs to be enabled.

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