Remove unused features to reduce dependency.
--- a/Cargo.toml	1970-01-01 08:00:00.000000000 +0800
+++ b/Cargo.toml	2019-05-28 14:26:39.734633297 +0800
@@ -53,7 +53,6 @@
 
 [dependencies.openssl]
 version = "^0.10"
-features = ["v102", "v110"]
 optional = true
 
 [dependencies.rand]