Remove mention of indirect dependencies: futures and tokio

These are not directly used in the crate, thus there is no need to mention them
in Cargo.toml. They are still pulled in my our dependencies (reqwest), but
that's cargo's job, not ours.

Cargo.toml

@@ -16,9 +16,7 @@ path            = "src/lib.rs"
 
 [dependencies]
 chrono          = { version = "0.4.19", features = ["serde"] }
-futures         = "0.3.8"
 serde           = { version = "1.0.118", features = ["derive"] }
 serde_json      = "1.0.60"
 reqwest         = { version = "0.10.9", features = ["json"] }
 url             = "2.2.0"
-tokio           = "0.2.24"