cargo: drop unused dependency on serde_derive

Since commit 70975d06 (remove unused macro_rules and macro_use,
2019-07-30) we no longer use the `serde_derive` crate.

Cargo.toml

@@ -17,7 +17,6 @@ itertools = "~0.7"
 log = "~0.4"
 percent-encoding = "^1.0"
 reqwest = "~0.9"
-serde_derive = "^1.0"
 
 chrono = { version = "~0.4", features = ["serde"] }
 error-chain = ">= 0.9, < 0.11"