Remove NodeSplitter and train_val_test_split

[huonw]

The NodeSplitter class and the train_val_test_split function seemingly have only been used in their own tests for quite a while now, specifically:

• 7f5cc7c from August 2018 seems to be the commit that removes the last use of NodeSplitter (by deleting demos/StellarGraph_examples.ipynb),
• 611d3f2 from September 2018 seems to be the commit that removes the last use of train_val_test_split (by deleting demos/node-classification-hinsage/generalization-tests.py).

Given these aren't used in our examples, it seems like we can probably remove them, and reduce how much code we need to maintain.

Replacement: manually cutting up a pandas data frame, or using sklearn.model_selection.train_test_split (most of our examples do the latter, now).

See: #885

[codeclimate]

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[codecov-io]

Codecov Report

Merging #887 into develop will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop    #887   +/-   ##
=======================================
  Coverage     85.1%   85.1%           
=======================================
  Files           49      49           
  Lines         5018    5018           
=======================================
  Hits          4268    4268           
  Misses         750     750           

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[kjun9]

Looks good to me