Remove Modin from Ray wheels. (#5647)

There are several reasons for this:

* We no longer support python2
* There should be only 1 way of installing Modin
  * Issue management on these wheels
* I have never heard of anyone using this feature
* It is rarely kept up to date
* Modin depends on specific versions of Ray because of past API changes

python/setup.py

@@ -90,13 +90,7 @@ class build_ext(_build_ext.build_ext):
             for name in filenames:
                 pyarrow_files.append(os.path.join(root, name))
 
-        # Make sure the relevant files for modin get copied.
-        modin_files = []
-        for (root, dirs, filenames) in os.walk("./ray/modin"):
-            for name in filenames:
-                modin_files.append(os.path.join(root, name))
-
-        files_to_include = ray_files + pyarrow_files + modin_files
+        files_to_include = ray_files + pyarrow_files
 
         # Copy over the autogenerated protobuf Python bindings.
         for directory in generated_python_directories: