[PHP7] Make default C++ extension .cxx

This is SWIG's default for every language except PHP, and now is a
good time to make this change so that once we drop PHP5 support, we'll
have the same default everywhere.  It's easy to override the default
with -cppext cxx to get the PHP5 behaviour.

Source/Modules/php.cxx

@@ -191,7 +191,6 @@ public:
 
   virtual void main(int argc, char *argv[]) {
     SWIG_library_directory("php");
-    SWIG_config_cppext("cpp");
 
     for (int i = 1; i < argc; i++) {
       if (strcmp(argv[i], "-prefix") == 0) {