Remove optional dependencies
--- a/Cargo.toml	1970-01-01 08:00:00.000000000 +0800
+++ b/Cargo.toml	2021-03-24 17:40:48.095164899 +0800
@@ -53,10 +53,6 @@
 [[example]]
 name = "test_synchronization"
 required-features = ["std"]
-[dependencies.parking_lot]
-version = "0.11"
-optional = true
-default_features = false
 [features]
 alloc = ["race"]
 default = ["std"]