Allow clippy warning manual-split-once

Not using `split_once` is a warning for `stable` but is not yet supported with our MSRV.
https://rust-lang.github.io/rust-clippy/master/index.html#manual_split_once

src/wallet/export.rs

@@ -95,6 +95,7 @@ impl FromStr for WalletExport {
     }
 }
 
+#[allow(clippy::manual-split-once)]
 fn remove_checksum(s: String) -> String {
     s.splitn(2, '#').next().map(String::from).unwrap()
 }