errmsg-utf8.txt nolonger uses charsets

Charset names in the 'languages' line are not used any more.

I think they should be just removed eventually, to avoid confusion.

All messages in errmsg-utf8.txt are in utf8 now.

Charset names should have been removed in MySQL-5.5 during: https://dev.mysql.com/worklog/task/?id=751

but it was forgotten forever