remove the crossbeam-channel dependency

Unused dependency reported by cargo-udeps.

Cargo.toml

@@ -11,14 +11,13 @@ edition = "2018"
 
 [features]
 default = ["std"]
-std = ["blake2s_simd/std", "crossbeam-channel", "num_cpus", "rayon"]
+std = ["blake2s_simd/std", "num_cpus", "rayon"]
 
 [dependencies]
 arrayref = "0.3.5"
 arrayvec = { version = "0.4.11", default-features = false, features = ["array-sizes-33-128"] }
 blake2s_simd = { version = "0.5.6", default-features = false }
 constant_time_eq = "0.1.3"
-crossbeam-channel = { version = "0.3.8", optional = true }
 num_cpus = { version = "1.10.1", optional = true }
 rayon = { version = "1.1.0", optional = true }