rpc: Remove getblockheader #29646

The getblockheader RPC with its true/false verbosity options returns the same content as getblock with verbosity 0/1. So this is an opportunity to remove some code very cleanly, as demonstrated here.

I looked for other issues or PRs that suggested this but wasn't successful, obviously let me know if I missed something.

Just looking for conceptual feedback for now. When that is positive I will first run getblockheader through a depecation cycle. If it is negative I will suggest some deduplication between the two RPCs instead.

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The getblockheader RPC with its true/false verbosity options returns the same content as getblock with verbosity 0/1.

I don't think that's the case. The following two commands on mainnet return different results:

$ ./src/bitcoin-cli getblockheader $(./src/bitcoin-cli getblockhash 830000) false
$ ./src/bitcoin-cli getblock $(./src/bitcoin-cli getblockhash 830000) 0

(What is a little confusing though is that the submitheader RPC doesn't care if there is more than 80 bytes passed, so feeding in a full block works as well. Context for other reviewers: #29478 (review))

Hm, weird, the docs are the same word for word as well. (Never mind, misread) I will need to dig a bit deeper and suggest some less drastic change/clarifications.