Remove SHELL_FILE setting?

This setting is completely untested, and it looks like it allows the user to override the JS shell file from the default of `src/shell.js` or `src/shell_minimal.js`.

These shell files used be split into two sections with `{{BODY}}` in the middle but it looks like this feature isn't needed either since both our shell files end when with `{{BODY}}` with no further content.

If we do have users of `-sSHELL_FILE` out there they would have to constantly work to keep their shell.js ABI in sync with all changes to emscripten, so it seems unlikely there are any.

Should I add a warning if they settings is used, and then remove it completely in a few releases?