Remove the `peer_disconnected` `no_connection_possible` flag

Remove the `peer_disconnected` `no_connection_possible` flag

Long ago, we used the `no_connection_possible` to signal that a
peer has some unknown feature set or some other condition prevents
us from ever connecting to the given peer. In that case we'd
automatically force-close all channels with the given peer. This
was somewhat surprising to users so we removed the automatic
force-close, leaving the flag serving no LDK-internal purpose.

Distilling the concept of "can we connect to this peer again in the
future" to a simple flag turns out to be ripe with edge cases, so
users actually using the flag to force-close channels would likely
cause surprising behavior.

Thus, there's really not a lot of reason to keep the flag,
especially given its untested and likely to be broken in subtle
ways anyway.