Dedup chain parsing (#2464)

### Description

Missed this by accident from #2249

### Drive-by changes

None

### Related issues

### Backward compatibility

_Are these changes backward compatible?_

Yes

_Are there any infrastructure implications, e.g. changes that would
prohibit deploying older commits using this infra tooling?_

None


### Testing

_What kind of testing have these changes undergone?_

Unit Tests

rust/agents/relayer/src/settings/mod.rs

@@ -246,11 +246,7 @@ impl FromRawConf<'_, RawRelayerSettings> for RelayerSettings {
             #[allow(deprecated)]
             raw.originchainname
         }
-        .map(|s| {
-            s.split(',')
-                .map(str::to_ascii_lowercase)
-                .collect::<Vec<_>>()
-        });
+        .map(parse_chains);
 
         if origin_chain_names.is_some() {
             warn!(
@@ -263,11 +259,7 @@ impl FromRawConf<'_, RawRelayerSettings> for RelayerSettings {
             #[allow(deprecated)]
             raw.destinationchainnames
         }
-        .map(|r| {
-            r.split(',')
-                .map(str::to_ascii_lowercase)
-                .collect::<Vec<_>>()
-        });
+        .map(parse_chains);
 
         if destination_chain_names.is_some() {
             warn!(
@@ -276,11 +268,7 @@ impl FromRawConf<'_, RawRelayerSettings> for RelayerSettings {
             );
         }
 
-        if let Some(relay_chain_names) = raw.relaychains.map(|r| {
-            r.split(',')
-                .map(str::to_ascii_lowercase)
-                .collect::<Vec<_>>()
-        }) {
+        if let Some(relay_chain_names) = raw.relaychains.map(parse_chains) {
             if origin_chain_names.is_some() {
                 err.push(
                     cwp + "originchainname",
@@ -400,3 +388,7 @@ impl FromRawConf<'_, RawRelayerSettings> for RelayerSettings {
 fn default_gasfraction() -> String {
     "1/2".into()
 }
+
+fn parse_chains(chains_str: String) -> Vec<String> {
+    chains_str.split(',').map(str::to_ascii_lowercase).collect()
+}