Remove mesa.flat namespace (#2091)

This doesn't seem to be used as often as the current simple namespace.

Author: rht

mesa/flat/__init__.py

@@ -2,30 +2,25 @@ def test_import():
     # This tests the new, simpler Mesa namespace. See
     # https://github.com/projectmesa/mesa/pull/1294.
     import mesa
-    import mesa.flat as mf
     from mesa.time import RandomActivation
 
     _ = mesa.time.RandomActivation
     _ = RandomActivation
-    _ = mf.RandomActivation
 
     from mesa.space import MultiGrid
 
     _ = mesa.space.MultiGrid
     _ = MultiGrid
-    _ = mf.MultiGrid
 
     from mesa.visualization_old.ModularVisualization import ModularServer
 
     _ = mesa.visualization_old.ModularServer
     _ = ModularServer
-    _ = mf.ModularServer
 
     from mesa.datacollection import DataCollector
 
     _ = DataCollector
     _ = mesa.DataCollector
-    _ = mf.DataCollector
 
     from mesa.batchrunner import batch_run