Prune down Database.column_placeholders() a bit

lib/nexrad/db.py

@@ -108,23 +108,7 @@ class Database():
         return Database(db)
 
     def column_placeholders(self, table, obj) -> list:
-        ctors = getattr(table, '__constructors__')
-
-        if ctors is None:
-            return ['?' for c in table.__columns__ if c != table.__key__]
-        if ctors is not None:
-            ret = list()
-
-            for c in table.__columns__:
-                if c == table.__key__:
-                    continue
-
-                if c in ctors:
-                    ret.append(ctors[c](getattr(obj, c)))
-                else:
-                    ret.append('?')
-
-            return ret
+        return ['?' for c in table.__columns__ if c != table.__key__]
 
     def add(self, obj):
         fn = getattr(obj, '__insert__')