rustfmt: Drop remaining util files from exclusion list

Author: tnull

rustfmt_excluded_files

@@ -72,6 +72,3 @@ lightning/src/routing/router.rs
 lightning/src/routing/scoring.rs
 lightning/src/routing/test_utils.rs
 lightning/src/routing/utxo.rs
-lightning/src/util/invoice.rs
-lightning/src/util/message_signing.rs
-lightning/src/util/string.rs