Remove unreachable exception handling in ip6.py

[crocogorical]

During packing of IP6 instances, it does not appear to be possible to raise an AttributeError during this section of code.

• The if statement on line 117 already reads self.data.sum
• No attributes are accessed by dpkt.in_cksum_done.

It is possible that self.data.sum could be read-only, which would raise an AttributeError when trying to write to it.

I cannot think of a way to generate this exception, so we should remove the code, or design a test for it.

[brifordwylie]

@crocogorical Feel free to merge at your convenience