build: remove the Python 2 check

[compnerd]

The Swift build is now fully migrated to Python 3. Although it is
backwards/forwards compatible, the build invokes everything with the
Python3 interpreter, and the Python 2 interpreter should be unused.
Remove the last reference to it.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

@swift-ci please clean test

[compnerd]

CC: @drexin @shahmishal

[gwynne]

🎉🎉

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 82e870b

[compnerd]

@shahmishal seems that the macOS builders are missing flake8 again?

FAIL: Swift(macosx-x86_64) :: Python/python_lint.swift (7422 of 13687)
******************** TEST 'Swift(macosx-x86_64) :: Python/python_lint.swift' FAILED ********************
Script:
--
: 'RUN: at line 8';   /Applications/Xcode-beta.app/Contents/Developer/usr/bin/python3 /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master/swift/utils/python_lint.py
--
Exit Code: 1

Command Output (stdout):
--

The flake8 and flake8-import-order Python packages are required for linting,
but these were not found on your system.

You can install these using:

    python -m pip install flake8
    python -m pip install flake8-import-order

For more help, see http://flake8.pycqa.org.


--

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 82e870b

[compnerd]

@swift-ci please test Linux platform

[shahmishal]

@swift-ci test macOS

[shahmishal]

@compnerd I fixed the issue with flake8 module, let me know if you see it again.

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 82e870b

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 82e870b

[CodaFi]

@swift-ci clean test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 82e870b

[compnerd]

@swift-ci please test Linux platform