Remove "3 pubkeys in top level multisig" limit

[dennisreimann]

This was introduced in #696 and it seems like an arbitrary limit.

@joemphilips What is the reason for this?

[joemphilips]

This limit is taken from original implementation in bitcoind
https://github.com/bitcoin/bitcoin/blob/71abee86dba8f311775104c518644c55c3e123cf/src/test/descriptor_tests.cpp#L440

Why you want to use bare multisig? IIUC it is deprecated and you should use wsh(multi()) or sh(multi()) or sh(wsh(multi()))

[dennisreimann]

This isn't a bare multisig, I'm coming across it in a wsh(sortedmulti(2,[4 PUBKEYS])) scenario when importing a wallet file containing that output descriptor in BTCPay Server.

But now that you hinted at that, I think the error isn't in the implementation here, but in our DerivationSchemeParser, which uses OutputDescriptor.Parse recursively, @NicolasDorier.

[dennisreimann]

@joemphilips I was able to fix this issue on our end — thanks for the pointer, you are correct!