ICS004: Remove last packet sent

[AdityaSripal]

closes: #1024
closes: #1015
closes: #1023

[crodriguezvega]

Nice! 🧹

[sangier]

If I understand correctly, the core idea here is that we can remove the lastPacketSent field because the following statements remain true:

1. 'Any packet sent to the channel with a packet sequence number greater than the lastPacketSent will be rejected until the upgrade is complete.'

   • This statement remains valid because, even without the lastPacketSent field, the transition to the FLUSHING state during the startFlushUpgradeHandshake operation effectively blocks the sendPacket function. As a result, no additional packets can be sent over this channel during the upgrade process.
   • Considering that we were storing the lastPacketSent when we were moving to FLUSHING, the system without lastPacketSent achieve the same result.

2. We still have a way to check if there are pending inflight packets.

   • pendingInflightPacket does not depend on lastPacketSent. Indeed is designed to monitor the pending in-flight packet verifying the presence/absence of packet commitment given that packet commitments are deleted only when the packet lifecycle is complete. Therefore, if a packet commitment exists on the sender's chain, it indicates that the packet lifecycle is still incomplete.

Am I missing something? If all the above is correct, IMO, removing lastPacketSent is safe.

[damiannolan]

Also closes #1015 as it won't be required after this PR 👍
Thanks @AdityaSripal ❤️