Remove remaining code to support macos9

This OS has been unsupported for over 20 years.  We stopped providing
macswig builds more than 20 years ago too:

https://sourceforge.net/projects/swig/files/macswig/

The required SIOUX library doesn't seem to be available anywhere
now either.

Closes #2323

Source/Swig/include.c

@@ -110,11 +110,7 @@ static List *Swig_search_path_any(int syspath) {
   assert(slist);
   filename = NewStringEmpty();
   assert(filename);
-#ifdef MACSWIG
-  Printf(filename, "%s", SWIG_FILE_DELIMITER);
-#else
   Printf(filename, ".%s", SWIG_FILE_DELIMITER);
-#endif
   Append(slist, filename);
   Delete(filename);
   

Source/Swig/misc.c

@@ -218,7 +218,7 @@ void Swig_filename_correct(String *filename) {
     if (fname[0] == '/' && fname[1] == '/')
       network_path = 1;
   }
-#if defined(_WIN32) || defined(MACSWIG)
+#if defined(_WIN32)
   /* accept Unix path separator on non-Unix systems */
   Replaceall(filename, "/", SWIG_FILE_DELIMITER);
 #endif

Source/Swig/swig.h

@@ -14,9 +14,7 @@
 #ifndef SWIG_SWIG_H
 #define SWIG_SWIG_H
 
-#ifndef MACSWIG
 #include "swigconfig.h"
-#endif
 
 #include <stdio.h>
 #include <string.h>

Source/Swig/swigfile.h

@@ -34,9 +34,7 @@ extern void   Swig_file_debug_set(void);
 
 /* Delimiter used in accessing files and directories */
 
-#if defined(MACSWIG)
-#  define SWIG_FILE_DELIMITER ":"
-#elif defined(_WIN32)
+#if defined(_WIN32)
 #  define SWIG_FILE_DELIMITER "\\"
 #else
 #  define SWIG_FILE_DELIMITER "/"