bomb fire launch torpedo: Don't disclose ship sinking in retreat

bomb fire launch torpedo: Don't disclose ship sinking in retreat

These commands report "sunk!" even when the ship survives the attack
but sinks during retreat.  bomb even reports where on the retreat the
ship sinks.  Has been that way since retreat was added in Chainsaw.

Report "sunk!" only when the attack sinks the ship directly.

Similar code exists for land units, but it doesn't report killings.
Change it anyway, to keep it consistent with the ship code.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>