remove IRIX support (closes bpo-31341) (#3310)

See PEP 11.
diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
index beadecf..37626e6 100644
--- a/Modules/socketmodule.c
+++ b/Modules/socketmodule.c
@@ -268,10 +268,8 @@
 #include <netdb.h>
 #endif
 
-/* Irix 6.5 fails to define this variable at all. This is needed
-   for both GCC and SGI's compiler. I'd say that the SGI headers
-   are just busted. Same thing for Solaris. */
-#if (defined(__sgi) || defined(sun)) && !defined(INET_ADDRSTRLEN)
+/* Solaris fails to define this variable at all. */
+#if defined(sun) && !defined(INET_ADDRSTRLEN)
 #define INET_ADDRSTRLEN 16
 #endif