Remove _purple_oscar_convert()

[MarkDoliner]

We can remove the _purple_oscar_convert() function from util.c/util.h for libpurple 3.0.0.

[rlaager]

For the record: I looked at this and said, "We can remove it now, as it's an internal symbol." So I looked into doing that and found it is still called (or at least exists in the source code, I just did a grep).

[MarkDoliner]

Yeah, I think it's still called in the four places where we read in our xml files. And the function isn't actually static... it just starts with an underscore and the documentation tells plugins not to use it.

But it's not really harming anything. I think it'd be good to leave it in there until 3.0.0 regardless, so anyone upgrading from 1.5.0 will have working AIM/ICQ accounts.