Revert "Add a rand-std feature"

Author: apoelstra

Cargo.toml

@@ -29,8 +29,7 @@ path = "src/lib.rs"
 unstable = []
 default = ["std"]
 fuzztarget = []
-std = []
-rand-std = ["rand/std"]
+std = ["rand/std"]
 recovery = []
 endomorphism = []
 lowmemory = []