fix unsafe HTML construction with createIcon

[stepankuzmin]

This PR removes IE support for icon creation and removes createIcon from the MapboxGeocoder prototype.

[ryanhamley]

This looks good. I understand the rationale for removing createIcon from index.js, but this does technically change the API. It's not a documented method, but I think removing it might still necessitate a major version change, so maybe it's better to leave it be.

[stepankuzmin]

Thanks Ryan!

As per discussion with @mapbox-danny we can hold on the merge until a major version update

[ryanhamley]

@stepankuzmin I think it's ok to revert the change that removes createIcon from the public API and just remove the offending IE11 code. That will resolve the open security ticket.

[stepankuzmin]

@ryanhamley sure, I've updated the PR

[mapbox-danny]

Thanks for adding the Promise.resolve fix too