Always use destroy reason NONE in circuit handshake code
Closes #2466 (closed).
This updates the relay circuit handshake code to always use DestroyReason::NONE, and adds a comment to Destroy in tor-cell saying that we should always use DestroyReason::NONE for new cells.
I didn't audit all DESTROY cells in arti so we still might be generating some somewhere with non-NONE reasons. I'll open a ticket for this.