sonar: Drop a redundant putship()

sonar: Drop a redundant putship()

The old retreat_ship() took care not to put its ship argument (it
still put other ships in a group retreat).  Callers put it
unconditionally to make the change to the ship permanent.

The current retreat code puts all ships it changes, rendering sona()'s
putship() redundant.  Drop it.

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