integers_stubs_js: removing +nat.js and --source-map

+nat.js is only useful when using the Num library.
It is useless (for this test), and source-map parameter is also useless for
running the test.

This removes some warning like:
```
warning: overriding primitive "set_to_zero_nat"
  old: +nat.js:50
  new: +nat.js:50
```

Source: https://ocsigen.org/js_of_ocaml/4.0.0/manual/overview

Author: dannywillems

src/lib/integers_stubs_js/test/dune

@@ -2,7 +2,7 @@
  (name test)
  (modes js)
  (js_of_ocaml
-  (flags +nat.js --source-map --pretty))
+  (flags --pretty))
  (libraries integers integers_stubs_js)
  (instrumentation
   (backend bisect_ppx))