volatility: use system Python 2

[iMichka]

Because we will remove the python@2 formula end of 2019.

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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[iMichka]

==> python -s /private/tmp/volatility--homebrew-virtualenv-20191214-60625-1xupx9k/target/bin/virtualenv -p python /usr/local/Cellar/volatility/2.6.1_2/libexec
python: can't open file '/private/tmp/volatility--homebrew-virtualenv-20191214-60625-1xupx9k/target/bin/virtualenv': [Errno 2] No such file or directory