Removed 3.5 map image message

Reasoning: 3.5 images and most other map images won't work in the editor. If you try to import a random image, it will give you an empty map. Only maps that were exported as images previously will work correctly.

core/assets/bundles/bundle.properties

@@ -391,7 +391,7 @@ editor.removeunit = Remove Unit
 editor.teams = Teams
 editor.errorload = Error loading file.
 editor.errorsave = Error saving file.
-editor.errorimage = That's an image, not a map.\n\nIf you want to import a 3.5/build 40 map, use the 'Import Legacy Map' button in the editor.
+editor.errorimage = That's an image, not a map.
 editor.errorlegacy = This map is too old, and uses a legacy map format that is no longer supported.
 editor.errornot = This is not a map file.
 editor.errorheader = This map file is either not valid or corrupt.

core/src/mindustry/io/MapIO.java

@@ -11,6 +11,7 @@ import mindustry.core.*;
 import mindustry.game.*;
 import mindustry.maps.*;
 import mindustry.world.*;
+import mindustry.world.blocks.environment.*;
 import mindustry.world.blocks.storage.*;
 
 import java.io.*;
@@ -204,6 +205,11 @@ public class MapIO{
                     }
                 }
             }
+
+            //default to stone floor
+            if(tile.floor() == Blocks.air){
+                tile.setFloorUnder((Floor)Blocks.stone);
+            }
         }
     }
 }