subs: Drop unused spy detection roll from unitsatxy()

subs: Drop unused spy detection roll from unitsatxy()

The die roll was added in 4.0.17 to hide spies from bombers.  However,
players could roll dice as often as they wanted, by typing '?' at the
target prompt.  The broken die roll is unreachable since commit
7688aed disallowed bombing of spies.  Drop it.

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