Remove field sig_figs from namecoin.Token

[webmaster128]

The interface namecoin.Token has a field sig_figs. However, all tokens on the weave chain are fixed to 9 fractional digits. Thus the field is probably unused and misleading.

[ethanfrey]

This now refers to the following file

https://github.com/iov-one/weave/blob/master/x/currency/codec.proto

I agree that this field is unused, both in client and server, and can be removed