removed not needed dipendency for persy feature

Cargo.toml

@@ -88,7 +88,7 @@ heed = { git = "https://github.com/timokoesters/heed.git", rev = "f6f825da7fb2c7
 [features]
 default = ["conduit_bin", "backend_sqlite"]
 backend_sled = ["sled"]
-backend_persy = ["persy","num_cpus"]
+backend_persy = ["persy"]
 backend_sqlite = ["sqlite"]
 backend_heed = ["heed", "crossbeam"]
 sqlite = ["rusqlite", "parking_lot", "crossbeam", "tokio/signal"]