build(js_of_ocaml): Remove --no-source-map from root dune file

as for some reason a duplicate flag was passed, causing failure:

href: https://github.com/ocaml-sf/learn-ocaml/actions/runs/12846419515/job/35821578447?pr=602
```
 #26 15.79 - File "src/app/dune", line 86, characters 7-30:
 #26 15.79 - 86 |  (name learnocaml_student_view)
 #26 15.79 -             ^^^^^^^^^^^^^^^^^^^^^^^
 #26 15.79 - (cd _build/default/src/app && /home/opam/.opam/5.1/bin/js_of_ocaml --no-source-map --no-source-map --opt=2 --enable=use-js-string --target-env=browser -o learnocaml_student_view.bc.js /home/opam/.opam/5.1/lib/cstruct/cstruct.js /home/opam/.opam/5.1/lib/bigstringaf/runtime.js ../ace-lib/ace_bindings.js learnocaml_student_view.bc-for-jsoo)
 #26 15.79 - js_of_ocaml: option '--no-source-map' cannot be repeated
 #26 15.79 - Usage: js_of_ocaml [COMMAND] …
 #26 15.79 - Try 'js_of_ocaml --help' for more information.
```
using js_of_ocaml 5.9.1

Author: erikmd

dune

@@ -12,7 +12,7 @@
            (ocamlopt_flags)
            (js_of_ocaml
             (compilation_mode whole_program)
-            (flags --no-source-map --opt=2 --enable=use-js-string --target-env=browser)))
+            (flags --opt=2 --enable=use-js-string --target-env=browser)))
 
   (dev     (flags (:standard -safe-string -w -32 -warn-error -a+8))
            (js_of_ocaml