Investigate removing es5 binaryen flag

[spotandjake]

It looks like es6 is now supported by js_of_ocaml so we could probably remove the special jsoo flags from binaryen cmake.

[spotandjake]

I was testing this in #137 and got a succesful build upstream of the grain repo here grain-lang/grain#2331

However I closed my pr as I couldn't get the test suite working, it seemed the test.bc.js file being outputted by our tests in both binaryen.ml and libbinaryen were not well formed emscrascript modules.