Remove redundant serde_json dependency

common-api/Cargo.toml

 itertools = "0.10"
 near-account-id = { version = "0.15", optional = true }
 serde = { version = "1", default-features = false, features = ["derive"] }
-serde_json = "1"
 thiserror = "1"
 uuid = { version = "1", optional = true }
 x25519-dalek = { version = "1", features = ["serde"] }