socketpair: drop redundant _WIN32 branch and include

`io.h` already included via `curl_setup.h`, the other headers are
already guarded off for Windows. `INADDR_LOOPBACK` fallback remains
a no-op on Windows.

Closes #20032

lib/socketpair.c

@@ -107,27 +107,20 @@ int Curl_socketpair(int domain, int type, int protocol,
 }
 #endif /* USE_SOCKETPAIR */
 #else /* !HAVE_SOCKETPAIR */
-#ifdef _WIN32
-/*
- * This is a socketpair() implementation for Windows.
- */
-#ifdef HAVE_IO_H
-#include <io.h>
-#endif
-#else
+
 #ifdef HAVE_NETDB_H
 #include <netdb.h>
 #endif
 #ifdef HAVE_NETINET_IN_H
-#include <netinet/in.h> /* IPPROTO_TCP */
+#include <netinet/in.h> /* for IPPROTO_TCP */
 #endif
 #ifdef HAVE_ARPA_INET_H
 #include <arpa/inet.h>
 #endif
+
 #ifndef INADDR_LOOPBACK
 #define INADDR_LOOPBACK 0x7f000001
-#endif /* !INADDR_LOOPBACK */
-#endif /* !_WIN32 */
+#endif
 
 #include "curlx/nonblock.h" /* for curlx_nonblock */
 #include "curlx/timeval.h"  /* needed before select.h */