(nullify_objects): Simply use all of bigcity_dchr. Using only parts

(nullify_objects): Simply use all of bigcity_dchr.  Using only parts
is confusing.
(bigcity_dchr): Fix initializer for d_maxpop.

(dchr): The previous revision accidentally changed maximum population
of sanctuaries, plains, sea and wasteland.  Revert the former two.
The latter are not used, but the new max population of zero makes more
sense, so keep it.