Amend RecvPacket to remove references to FlushStatus

[DimitrisJim]

Summary

Receive packet needs to be amended to match the updated spec:

https://github.com/cosmos/ibc/blob/256205c409a4bbef1bad9f3f2e4d8b444e4c5e17/spec/core/ics-004-channel-and-packet-semantics/README.md?plain=1#L665

Mainly involves dropping the references to in the channel state check (and adding a check for FLUSHING) and amending the check for
the packet sequence.

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