[emscripten] Drop `-sWASM=0` variant build (#7995)

There is now just one emscripten target called `binaryen_js`. The
settings for these two targets were almost identical except for.

1. Some JS_OF_OCAML specific stuff which is kept for the new unified
target.
2. `-sASSERTIONS` was being forced (See #2507). I dropped this in favor
of doing a debug build in testing where this is enabled by default.