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/ray/__init__.py

@@ -53,9 +53,6 @@ except ImportError as e:
             e.args += (helpful_message, )
     raise
 
-modin_path = os.path.join(os.path.abspath(os.path.dirname(__file__)), "modin")
-sys.path.append(modin_path)
-
 from ray._raylet import (
     ActorCheckpointID,
     ActorClassID,