This removes a bunch of potentially new error handling in writers
and the checks were kinda useless anyway - in normal operation we
unwrap()ed anyway, and we're gonna want to use the serializtion
framework for ChannelMonitor/ChannelManager serialization, which
may generate things larger than 64KB anyway.
I think this might have been my fault due to faulty review
feedback, but fuzzer caught trivial crash here where you try to
send a ping message larger than 16KB (but smaller than the
max-length 64KB) and you crash as message serialization is unwrap()
