Deprecate non HX_SMART_STRINGS paths

Having to keep two code paths for text encoding stuff has been annoying me recently, I have two questions.

1. What is the `NO_HX_SMART_STRINGS` really for? I'm assuming this was just to switch back to the old code in case there were bugs in the new dual encoding system.
2. Is anyone actually using the non smart strings mode, if so, why? I'm assuming if anyone is they ran into a bug, disabled unicode string, and haven't touched it since.

It would be nice if for Haxe 5 we could stop supporting the fallback mode, any new api wouldn't need to support both that way.

Start by locating the NO_HX_SMART_STRINGS option and all code paths controlled by it, then determine whether the fallback is still used and why. Done would require an agreed Haxe 5 deprecation or removal plan, including the effect on existing APIs and any needed migration guidance.

Written by the indexing model from the issue text.