wasm-emscripten-finalize: Remove __start/__stop_em_js exports (#4870)
wasm-emscripten-finalize: Remove __start/__stop_em_js exports (#4870)

We already remove `__start_em_asm` and `__stop_em_asm`.  This change
is needed since I want to start exporting `__start_em_js` and
`__stop_em_js` from emscripten without causing regressions.

As a followup I'm planning on moving all of the em_js and em_asm
stripping code it PostEmscripten.cpp.
WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen