BIP119: remove James O'Beirne as co-author

I discussed removal with @jamesob at the beginning of the summer, as his focus is elsewhere these days. Original context on his addition is here https://github.com/bitcoin/bips/pull/1482. Since the BIP editor community is more active these days, I'd like to see the addition of an editors/champion/draft-team metadata, and would love to add a volunteer or two to the BIP should it need any edits for that role.

Specification