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
```

MarcoFalke:
  ACK c514a4f59a7430f05dbe20465ddf4ca323329f1e 🔄

ninja all check check-functional