windows: cleanup wrong and duplicated socket validity helpers

lib/core/libwebsockets.c

lib/core/pollfd.c

lib/core/private.h

lib/event-libs/libuv/libuv.c

lib/libwebsockets.h

lib/plat/lws-plat-unix.c

lib/roles/cgi/cgi-server.c

lib/roles/http/client/client-handshake.c

lib/roles/http/server/parsers.c

lib/roles/http/server/server.c

lib/roles/listen/ops-listen.c

lib/roles/ws/ops-ws.c

lib/tls/tls.c