Fallback all unknown AI types to "classic"

"classic" is the only AI available in fc21 anyway. It's better to load the
game slightly wrong than to fail to load it because of an unsupported AI type.
See #746.

common/ai.cpp

@@ -119,15 +119,10 @@ const char *ai_type_name_or_fallback(const char *orig_name)
     return orig_name;
   }
 
-  if (!strcmp("threaded", orig_name)) {
-    struct ai_type *fb;
-
-    fb = ai_type_by_name("classic");
-
-    if (fb != NULL) {
-      // Get pointer to persistent name of the ai_type
-      return ai_name(fb);
-    }
+  auto fb = ai_type_by_name("classic");
+  if (fb != nullptr) {
+    // Get pointer to persistent name of the ai_type
+    return ai_name(fb);
   }
 
   return NULL;