chore: Remove serde dependency

rust-elements has the default feature "json-contract" which depends on
serde_json. We don't need this feature in rust-simplicity.

Cargo.toml

@@ -14,7 +14,7 @@ rust-version = "1.63.0"
 default = ["elements"]
 elements = ["dep:elements", "bitcoin"]
 test-utils = ["simplicity-sys/test-utils"]
-serde = ["dep:serde", "bitcoin/serde", "elements/serde"]
+serde = ["dep:serde", "bitcoin/serde", "elements/serde", "elements/json-contract"]
 
 [lib]
 name = "simplicity"
@@ -24,7 +24,7 @@ path = "src/lib.rs"
 bitcoin = { version = "0.32.0", optional = true }
 bitcoin-miniscript = { package = "miniscript", version = "12.0.0" }
 byteorder = "1.3"
-elements = { version = "0.25.0", optional = true }
+elements = { version = "0.25.0", optional = true, default-features = false }
 hashes = { package = "bitcoin_hashes", version = "0.14" }
 hex = { package = "hex-conservative", version = "0.1.1" }
 santiago = "1.3"