Currently our wallet code will treat bare multisig outputs (meaning scriptPubKeys with multiple public keys + `OP_CHECKMULTISIG` operator in it) as ours without the user asking for it, as long as all private keys in it are in our wallet.

This is a pointless feature. As it only works when all private keys are in one place, it's useless compared to single key outputs (P2PK, P2PKH, P2WPKH, P2SH-P2WPKH), and worse in terms of space, cost, UTXO size, and ability to test (due to lack of address format for them).

Furthermore, they are problematic in that producing a list of all `scriptPubKeys` we accept is not tractable (it involves all combinations of all public keys that are ours). In further wallet changes I'd like to move to a model where all scriptPubKeys that are treated as ours are explicit, rather than defined by whatever keys we have. The current behavior of the wallet is very hard to model in such a design, so I'd like to get rid of it.

I think there are two options:
* Remove it entirely (do not ever accept bare multisig outputs as ours, unless watched)
* Only accept bare multisig outputs in situations where the P2SH version of that output would also be acceptable

This PR implements the first option. The second option was explored in #12874.