Java: rename delete function to _delete and make private.

At the moment, Java classes are created with a special `delete`
function that is in charge of clearing the Rust object, e.g.:

```
public synchronized void delete() {
    if (mNativeObj != 0) {
        do_delete(mNativeObj);
        mNativeObj = 0;
   }
}
```

Delete is quite a useful name to have (I use it in my API) and
this function being called this way is confusing, but also
needlessly limits the API.

Fixes Dushistov#350

macroslib/src/java_jni/fclass.rs

@@ -442,7 +442,7 @@ May be you need to use `private constructor = empty;` syntax?",
         writeln!(
             file,
             r#"
-    public synchronized void delete() {{
+    private synchronized void _delete() {{
         if ({rust_self_name} != 0) {{
             do_delete({rust_self_name});
             {rust_self_name} = 0;
@@ -451,7 +451,7 @@ May be you need to use `private constructor = empty;` syntax?",
     @Override
     protected void finalize() throws Throwable {{
         try {{
-            delete();
+            _delete();
         }}
         finally {{
              super.finalize();