investigate: channel state changes two times to AWAITING_UNERLATERAL on force close

[m-schmoock]

Issue and Steps to Reproduce

When a channel is closed unilaterally the channel changes the state to AWAITING_UNERLATERAL two times, which means
the second time the channel is already in that state.

This was discovered when implementing tests for #4020

See 160c564 for the 'fix' to suppress the duplicated event notification.

[m-schmoock]

Since we currently save the channel state to the database on each state change (for good reason), we can't skip duplicated updates without selectively checking the database state. The duplicated state changes came from the onchain watchers in onchain_control.c and thechannel_fail_permanent that is used throughout the code.

Addressing this seems over-engineering to me, so we are better off by simply suppressing reaching out duplicated events to the API.