timing.core: simplify hashCode, equals and toString of AbstractCalledFunction

timing.core: simplify hashCode, equals and toString of AbstractCalledFunction

* hashcode now uses Objects.hash
* equals new uses and Object.equals
* toString now avoids a useless new String()

Change-Id: Ib409d1eee45a17b917f334d93e52a6845bcd2954
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/82308
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Hudson CI