Small performance improvement in SerialExecutor trough preventing exceptions.

git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1529831 13f79535-47bb-0310-9956-ffa450edef68
Removed unused import.

git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1529834 13f79535-47bb-0310-9956-ffa450edef68