Remove confusing and useless "unexpected version" warning #20054

It is useless because it isn't displayed for most users:

• It isn't displayed in normal operation (because the validation debug category is disabled by default)
• It isn't displayed for users that sync up their nodes intermittently, e.g. once a day or once a week (because it is disabled for IBD)
• It is only displayed in the debug log (as opposed to the versionbits warning, which is displayed more prominently)

It is confusing because it doesn't have a use case:

Despite the above, if a user did see the warning, it would most likely be a false positive (like it has been in the past). Even if it wasn't, there is nothing they can do about it. The only thing they could do is to check for updates and hope that a fixed version is available. But why would the user be so scrupulously precise in enabling the warning and reading the log, but then fail to regularly check update channels for updated software?

Related #19898.

The rationale makes sense for not displaying it to users, but we don't display it to users anymore after 19898 -- it's now a debug log option, so the justification for removing it should be that it doesn't assist with development or debugging or performance tuning or similar, I think.

FWIW, I've found it very slightly useful in that it triggered when I was working on signet soft-fork activation code and noticed those errors appearing, which revealed that kalle's node was trying to activate the testdummy soft-fork when it shouldn't have. That doesn't seem very useful though, so weak Concept ACK.

was trying to activate the testdummy soft-fork

The existing versionbits warning is not touched by this patch. If a miner is trying to activate the testdummy soft-fork, a warning will be shown to all users when the presumed soft-fork has activated. (Both before and after this patch). I understand that the 100-block warning will hit earlier than the vb-activated warning, but it shouldn't be the responsibility of the Bitcoin Core validation code to help debug miner software. A simple RPC python script (or even a hidden test-only RPC) should be less invasive and straightforward.

ACK 0000a0c7e9e4e7c1afafe6ef75b7624f4c573190

• Benefit: Less confusion. -14 LOC.
• Cost: None.
• Cost-benefit verdict: 🎉

ACK 0000a0c

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Conflicts

No conflicts as of last run.

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It did have a use case:

users scrupulously precise in enabling the warning and reading the log, but who fail to regularly check update channels for updated software