doc: clarify libbitcoinkernel usage in libraries design #34042

The libraries design document was out of sync with the current bitcoinkernel implementation and its consumers. It claimed that libbitcoin_node links against libbitcoin_kernel and that the kernel library only depends on consensus, crypto, and util, which is not true for the current CMake targets.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/34042.

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Conflicts

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• #28690 (build: Introduce internal kernel library by sedited)

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This has been discussed in #28690, which introduces a libbitcoin_kernel target. In the past the design doc was understood as a goal to work towards. I think we should finally decide whether to move forward with #28690, and if not, accept the changes you laid out here.