simplify kvdb error types (#8924)

util/error/src/lib.rs

@@ -25,7 +25,6 @@ extern crate error_chain;
 extern crate ethereum_types;
 extern crate rlp;
 extern crate rustc_hex;
-extern crate kvdb;
 
 use std::fmt;
 use rustc_hex::FromHexError;
@@ -63,10 +62,6 @@ error_chain! {
 		UtilError, ErrorKind, ResultExt, Result;
 	}
 
-	links {
-		Db(kvdb::Error, kvdb::ErrorKind);
-	}
-
 	foreign_links {
 		Io(::std::io::Error);
 		FromHex(FromHexError);