Make TX_WITNESS_STRIPPED its own rejection reason

Previously, TX_WITNESS_MUTATED could be returned during transaction validation
for either transactions that had a witness that was non-standard, or for
transactions that had no witness but were invalid due to segwit validation
rules.

However, for txid/wtxid-relay considerations, net_processing distinguishes the
witness stripped case separately, because it affects whether a wtxid should be
able to be added to the reject filter. It is safe to add the wtxid of a
witness-mutated transaction to the filter (as that wtxid shouldn't collide with
the txid, and hence it wouldn't interfere with transaction relay from
txid-relay peers), but it is not safe to add the wtxid (== txid) of a
witness-stripped transaction to the filter, because that would interfere with
relay of another transaction with the same txid (but different wtxid) when
relaying from txid-relay peers.

Also updates the comment explaining this logic, and explaining that we can get
rid of this complexity once there's a sufficient deployment of wtxid-relaying
peers on the network.

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