Merge rust-bitcoin#4297: chore: remove unused serde_json dep

6ba0e46 chore: remove unused serde_json dep (lfg2)

Pull request description:

  checked by [shear](https://github.com/boshen/cargo-shear)
  ```
  bitcoin_hashes -- hashes/Cargo.toml:
    serde_json

  Fixed 1 dependencies!
  ```

  cargo +nightly build  in local env is ok, local cargo.lock  is ok(not include serde_json) so no need update cargo.lock
  ```
  [[package]]
  name = "bitcoin_hashes"
  version = "0.16.0"
  dependencies = [
   "bitcoin-internals",
   "hex-conservative 0.3.0",
   "serde",
   "serde_test",
  ]
  ```

  but check CI seems need update
  `error: the lock file /home/runner/work/rust-bitcoin/rust-bitcoin/Cargo.lock needs to be updated but --locked was passed to prevent this
  If you want to try to generate the lock file without accessing the network, remove the --locked flag and use --offline instead.`

ACKs for top commit:
  Kixunil:
    ACK 6ba0e46
  apoelstra:
    ACK 6ba0e46; successfully ran local tests

Tree-SHA512: 68a444e05876733af9606d4489ca30a18c9d3439dedf932e40db67dcfdb5ae40ab0f8cf937146f6b1ef92edc626dcb27a441b63c283e6945aa8071500bcc43a4

Author: apoelstra

Cargo-minimal.lock

@@ -149,7 +149,6 @@ dependencies = [
  "bitcoin-internals",
  "hex-conservative 0.3.0",
  "serde",
- "serde_json",
  "serde_test",
 ]
 

Cargo-recent.lock

@@ -148,7 +148,6 @@ dependencies = [
  "bitcoin-internals",
  "hex-conservative 0.3.0",
  "serde",
- "serde_json",
  "serde_test",
 ]
 

hashes/Cargo.toml

@@ -28,7 +28,6 @@ serde = { version = "1.0", default-features = false, optional = true }
 
 [dev-dependencies]
 serde_test = "1.0"
-serde_json = "1.0"
 
 [package.metadata.docs.rs]
 all-features = true