This reverts commit b5fce696c7 and fixes
to NEWS and @since that came later.
These functions are pretty trivial and their functionality can be
obtained with asprintf() and snprintf. The first is not available only
on windows, but there's an implementation for that one on Evil, that
should be used instead.
eina_str_convert() is GUARANTEED to break when doing any such encoding (eg. UTF16->UTF8). I don't know who added the original function, but this is very bad, and we should almost certainly deprecate eina_str_convert() so people are not surprised when they are unable to convert strings as expected.