Remove slab dependency from time feature

Author: Bryan Donlan

tokio/Cargo.toml

@@ -83,7 +83,7 @@ stream = ["futures-core"]
 sync = ["fnv"]
 test-util = []
 tcp = ["lazy_static", "mio"]
-time = ["slab"]
+time = []
 udp = ["lazy_static", "mio"]
 uds = ["lazy_static", "libc", "mio", "mio-uds"]
 
@@ -101,7 +101,7 @@ memchr = { version = "2.2", optional = true }
 mio = { version = "0.6.20", optional = true }
 num_cpus = { version = "1.8.0", optional = true }
 parking_lot = { version = "0.11.0", optional = true } # Not in full
-slab = { version = "0.4.1", optional = true } # Backs `DelayQueue`
+slab = { version = "0.4.1", optional = true }
 tracing = { version = "0.1.16", default-features = false, features = ["std"], optional = true } # Not in full
 
 [target.'cfg(unix)'.dependencies]