build: enable `-Wunused-const-variable` #36275

Split out of #36167.

Fixes:

/root/bitcoin/src/leveldb/db/dbformat.h:42:18: warning: ‘leveldb::config::kMaxMemCompactLevel’ defined but not used [-Wunused-const-variable=]
   42 | static const int kMaxMemCompactLevel = 2;
      |                  ^~~~~~~~~~~~~~~~~~~
/root/bitcoin/src/leveldb/db/dbformat.h:28:18: warning: ‘leveldb::config::kL0_CompactionTrigger’ defined but not used [-Wunused-const-variable=]
   28 | static const int kL0_CompactionTrigger = 4;
      |                  ^~~~~~~~~~~~~~~~~~~~~
In file included from /root/bitcoin/src/leveldb/db/dbformat.h:13:
/root/bitcoin/src/leveldb/include/leveldb/db.h:19:18: warning: ‘leveldb::kMinorVersion’ defined but not used [-Wunused-const-variable=]
   19 | static const int kMinorVersion = 22;
      |                  ^~~~~~~~~~~~~
/root/bitcoin/src/leveldb/include/leveldb/db.h:18:18: warning: ‘leveldb::kMajorVersion’ defined but not used [-Wunused-const-variable=]
   18 | static const int kMajorVersion = 1;
      |                  ^~~~~~~~~~~~~

In file included from /home/runner/work/_temp/src/init/common.cpp:11:
/home/runner/work/_temp/src/logging.h:66:24: error: ‘BCLog::RATELIMIT_MAX_BYTES’ defined but not used [-Werror=unused-const-variable=]
   66 |     constexpr uint64_t RATELIMIT_MAX_BYTES{1_MiB}; // maximum number of bytes per source location that can be logged within the RATELIMIT_WINDOW
      |                        ^~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors

etc.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36275.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #36182 (fees: return block_policy fee rate estimate when mempool_policy is not ready by ismaelsadeeq)
• #35322 (logging: streamline Logger state and drop redundant methods by ryanofsky)
• #34038 (logging: replace -loglevel with -trace, expose trace logging via RPC by ajtowns)
• #31507 (build: Use clang-cl to build on Windows natively by hebasto)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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🚧 At least one of the CI tasks failed. _{Task lint: https://github.com/bitcoin/bitcoin/actions/runs/35114035836/job/104854768994} _{LLM reason (✨ experimental): CI failed because the lint “subtree” check detected that a subtree directory was touched without a corresponding subtree merge.}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

</details>

Maybe leave in draft while this conflicts with a backport?