Remove unnecessary whitespaces before end of line

Some python codes contained unnecessary whitespaces before end of line. I think it should be removed to avoid any further conflict. This pull request does not change any logic.

I removed these whitespaces from the following files:

    ./examples/school/categories.py
    ./examples/school/words.py
    ./examples/semantics/syn2sem.py
    ./nltk/app/collocations_app.py
    ./nltk/app/concordance_app.py
    ./nltk/collocations.py
    ./nltk/compat.py
    ./nltk/corpus/reader/framenet.py
    ./nltk/inference/discourse.py
    ./nltk/inference/prover9.py
    ./nltk/metrics/association.py
    ./nltk/metrics/segmentation.py
    ./nltk/parse/generate.py
    ./nltk/parse/util.py
    ./nltk/sem/boxer.py
    ./nltk/stem/porter.py
    ./nltk/tag/mapping.py
    ./nltk/tokenize/treebank.py
    ./pip-install.py
    ./setup.py