Remove check which makes us sometimes never send closing_signed

Remove check which makes us sometimes never send closing_signed

This is the case pointed out by nayuta-gondo at
https://github.com/lightningnetwork/lightning-rfc/issues/499#issuecomment-438623208
though this doesn't actually solve the issue of ensuring we have a
consistent fee view when we start shutdown processing. There isn't
a clear solution to that however without adding additional state
tracking in Channel.

This also removes an associated test that tests for the correct
behavior (but didn't consider the bug) as we no longer behave
correctly. This should be fine as we'll be removing all the
update_fee garbage with option_simplified_commitment anyway.