Make UTF-8 the only one supported language encoding

In-Portal always allowed to enter custom charset for each language you've add. However from time, when we implemented this feature a lot has changed and now UTF-8 is major charset that supports all of them together.

I propose to remove Charset from Language settings and always use UTF-8.

I'm proposing this because recently I had a need to do some symbol conversions in OnAfterConfigRead event, which is called before actual charset from a language is detected and mb_internal_encoding is set.

For backwards compatibility we still can support "one-per-system" charset setting in config.php (like we do now with database), but not "each language have it's own charset" system, that is bad.