[OE-core] sip.bbclass?

[alexander.kanavin at linux.intel.com]

> Nothing in OE-Core uses extrenalsrc directly but we'd still want
> something like that in the core so I don't think the rule is absolute.
> What we wouldn't want to see is multiple layers maintaining different
> and potentially incompatible versions of a class file.
>
> That said, I think there are a few that we probably could clean up.

As for sip.bblclass, nothing is using it in oe-core or meta-oe. So I just
removed it as a part of oe-core py3 patchset, because it's doing some
python 2 based magic, and I can't figure out what exactly without any
recipes to test with.

Alex