Remove serde_json stuff from the metadata RPC (#833)

The metadata call does not work anymore with JSON and just returns an
opaque blob.

substrate/core/rpc/Cargo.toml

@@ -18,7 +18,6 @@ substrate-primitives = { path = "../primitives" }
 sr-primitives = { path = "../sr-primitives" }
 sr-version = { path = "../sr-version" }
 tokio = "0.1.7"
-serde_json = "1.0"
 
 [dev-dependencies]
 assert_matches = "1.1"