Remove futures-core dependency from futures-io

Author: taiki-e

futures-io/Cargo.toml

@@ -15,11 +15,10 @@ The `AsyncRead` and `AsyncWrite` traits for the futures-rs library.
 name = "futures_io"
 
 [features]
-std = ["futures-core-preview/std"]
 default = ["std"]
+std = []
 
 [dependencies]
-futures-core-preview = { path = "../futures-core", version = "=0.3.0-alpha.16", default-features = false }
 
 [dev-dependencies]
 futures-preview = { path = "../futures", version = "=0.3.0-alpha.16" }