Wait to free the holding cell during channel_reestablish handling

Wait to free the holding cell during channel_reestablish handling

When we process a `channel_reestablish` message we free the HTLC
update holding cell as things may have changed while we were
disconnected. However, some time ago, to handle freeing from the
holding cell when a monitor update completes, we added a holding
cell freeing check in `get_and_clear_pending_msg_events`. This
leaves the in-`channel_reestablish` holding cell clear redundant,
as doing it immediately or is `get_and_clear_pending_msg_events` is
not a user-visible difference.

Thus, we remove the redundant code here, substantially simplifying
`handle_chan_restoration_locked` while we're at it.