fairland: Tweak rural iron, fert, oil for simplicity

The previous commit's interpolation tables reproduce the historical
linear functions faithfully.  This exact fit requires an awkward data
point in each of the tables for iron fertility and oil.  Drop them.

No double-valued data points remain.  Adjust struct resource_point
member @res to int.

This results in slightly less rural iron (33 elevations down one
point), fertility (10 elevations down one point) and oil (14
elevations down one point, 5 down two points).  Only resource values
less than 100 are affected.

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