Avoid spurious ocamlfind warning when not generating Graphics lib

[craigfe]

The dune file for the js_of_ocaml Graphics library is conditional on the availability of the graphics library, as determined by querying ocamlfind. This commit passes -qe to the OCamlfind query to avoid a redundant warning when graphics is not available:

$ dune build
       ocaml (internal)
ocamlfind: Package `graphics' not found

(This warning is visible to all packages that vendor js_of_ocaml.)

[craigfe]

At a higher level, it seems odd to require ocamlfind here at all, since ocamlfind is only a dep-opt of js_of_ocaml-compiler. There's a different error for not having an available ocamlfind:

$ dune build
       ocaml (internal)
sh: line 1: ocamlfind: command not found

What's the desired behaviour here when ocamlfind is not available?

[hhugo]

Ideally, we should not call ocamlfind our-self. It was added some time ago to work around an issue with optional libraries 6e80150.

Do you want to try remove it and see if the CI is happy ?

[hhugo]

alternative fix in #1118

[hhugo]

I've removed the explicit call to ocamfind