Clean up unused methods

[kmax12]

Made minor changes that clean up the code. No api changes.

Changes

• removed Relationship.get_other_variable because the method wasn't used
• removed Relationship.get_other_entity because it was only used one. Added logic to Entity.related_instances instead
• removed Relationship.get_entity_variable because it was only used one. Added logic to Entity.related_instances instead
• removed Relationship._get_link_variable_name and EntitySet. gen_relationship_var because they did the same thing and it isn't really a method of either of those classes. Created method get_relationship_variable_id in our utils file instead.
• removed unused methods _check_variable_list, _check_variable , and _check_entity from wrangle.py
• remove training_window_is_dict handling in related instances that is not possible
• removed unused methods Entity.is_child_of and Entity.is_parent_of
• remove unused method EntitySet.get_relationships
• remove unused method make_index_variable_name
• remove unused class RedirectStdStreams
• remove invalid property Discrete.percent_unique. this property access other properties that are no longer defined on Variable

[rwedge]

Did the training_window_is_dict change make it into the PR?

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c7bf678). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #293   +/-   ##
=========================================
  Coverage          ?   94.73%           
=========================================
  Files             ?       71           
  Lines             ?     7668           
  Branches          ?        0           
=========================================
  Hits              ?     7264           
  Misses            ?      404           
  Partials          ?        0

Impacted Files	Coverage Δ
featuretools/utils/wrangle.py	79.77% <ø> (ø)
featuretools/entityset/relationship.py	95.23% <ø> (ø)
...turetools/computational_backends/pandas_backend.py	94.11% <100%> (ø)
featuretools/entityset/entityset.py	93.75% <100%> (ø)
featuretools/utils/gen_utils.py	74.13% <100%> (ø)
...computational_backends/calculate_feature_matrix.py	96.96% <100%> (ø)

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[kmax12]

you're right, it didn't. just fixed

[rwedge]

Looks good!