[Chainstate] Drop BurnchainDB::get_burnchain_header()

[jcnelson]

This function incorrectly obtains a single burnchain block header by height, but there can be multiple rows. Fortunately, it's only present on rarely-used paths in the chains coordinator (but they're all involved with handling Bitcoin reorgs). This method needs to get reworked.

[blockstack-devops]

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