Remove code that sends htlc commit sigs to lightningd which then get thrown away

[ddustin]

Long ago code was written to send our htlc sigs to lightningd to store or do something with. Lightningd currently does nothing with them.

Removing this code would be a win

Rusty's comment:
Hmm, I chased this down and it seems that we send lightningd the commit sigs for our commit tx and it never uses them. A separate patch to remove that would be a win! (I was confused looking at this, thinking it surely needed the htlc sigs for the other inflights, but no...)

Originally posted by @rustyrussell in #6253 (comment)