Load subspawn builtins by default on non-Windows

The logic in subspawn is much more complete and passes more specs,
so this is a first step toward using it by default on all
platforms where it works.

core/src/main/java/org/jruby/Ruby.java

@@ -544,12 +544,8 @@ private Ruby(RubyInstanceConfig config) {
 
         // Done booting JRuby runtime
         runtimeIsBooted = true;
-        if ("true".equals(System.getenv("USE_SUBSPAWN"))) {
-            if (Platform.IS_WINDOWS) {
-                LOG.warn("env USE_SUBSPAWN=true is unsupported on Windows at this time");
-            } else {
-                getLoadService().require("subspawn/replace-builtin");
-            }
+        if (!Platform.IS_WINDOWS) {
+            getLoadService().require("subspawn/replace-builtin");
         }
     }