[commit/Ada] delete ada-lang.c:extract_string and use target_read_string instead

[Joel Brobecker]

> I don't know what the Ada compiler emits here, but offhand it seems
> like this needs a conversion to the host charset.
> I'd imagine in most cases everything is ASCII-compatible and therefore
> the problem never manifests.

Yes, it is in ascii. Can I get away without doing a conversion, or
would it be better if I did? The string just contains a symbol name.

-- 
Joel