Merge #19202: log: remove deprecated `db` log category

c514a4f59a7430f05dbe20465ddf4ca323329f1e doc: release note for `db` log category removal (Jon Atack)
4c0c89307dabbf51a32551471c54966ddf7c5bc3 log: remove deprecated `db` log category (Jon Atack)

Pull request description:

  The `db` log category was renamed to `walletdb` (like `coindb`) in #17410 and its upcoming removal announced in the 0.20 release notes.

  ```
  - The `-debug=db` logging category has been renamed to
    `-debug=walletdb` to distinguish it from `coindb`.  The `-debug=db`
    option has been deprecated and will be removed in the next major
    release.  (#17410)
  ```

  This PR removes the warning and reverts to the usual behavior for an unrecognised log category.
  ```
  $ bitcoin-cli logging '["db"]'
  error code: -8
  error message:
  unknown logging category db
  ```
  ```
  $ ./src/bitcoind -debug=db
  Warning: Unsupported logging category -debug=db.
  2020-06-07T15:30:45Z Bitcoin Core version v0.20.99.0-4c0c89307d (debug build)
  2020-06-07T15:30:45Z Warning: Unsupported logging category -debug=db.
  2020-06-07T15:30:45Z Assuming ancestors of block 0000000000000000000f2adce67e49b0b6bdeb9de8b7c3d7e93b21e7fc1e819d have valid signatures.
  2020-06-07T15:30:45Z Setting nMinimumChainWork=00000000000000000000000000000000000000000e1ab5ec9348e9f4b8eb8154
  2020-06-07T15:30:45Z Using the 'sse4(1way),sse41(4way),avx2(8way)' SHA256 implementation
  2020-06-07T15:30:45Z Using RdSeed as additional entropy source
  ```

ACKs for top commit:
  MarcoFalke:
    ACK c514a4f59a7430f05dbe20465ddf4ca323329f1e 🔄

Tree-SHA512: fd62fd7ae0dc65446ba4401d75b4047e055396a33f7f1b176e79a7753250aec2a474ae604163d3f7e68710443c0ed2f45e44435d15f35612d794807e2142d5a3

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