jenkins: fix shellcheck linter CI (nodejs#3115)

Recently released shellcheck 0.9.0 adds a new check:
```
SC2317 (info): Command appears to be unreachable. Check usage (or
ignore if invoked indirectly).
```

Fix the one instance of this that is causing the CI to now fail with
the updated version of shellcheck.

Refs: koalaman/shellcheck@642ad86

jenkins/scripts/select-compiler.sh

@@ -60,7 +60,6 @@ case $NODE_NAME in
         return
         ;;
     esac
-    return
     ;;
 esac