Remove use of method unsupported by Java 6

hikaricp-common/src/test/java/com/zaxxer/hikari/TestConnections.java

@@ -139,7 +139,7 @@ public class TestConnections
       }
       finally {
          ds.close();
-         System.getProperties().remove("com.zaxxer.hikari.housekeeping.periodMs", "100");
+         System.getProperties().remove("com.zaxxer.hikari.housekeeping.periodMs");
       }
    }