Remove Assertion That Pay-To-Pubkey (Unhashed) Still Used In Coinbase Txes

_includes/guide_transactions.md:

* Assertions that pay-to-public-key (unhashed) was "used in all coinbase
  transactions" and "is more convenient" have been removed: the first
  statement because its provably untrue; the second because it's debatable
  (spending unhashed keys requires fewer bytes, so it might be more
  convenient).  Based on feedback from @TierNolan.  Thanks!