Removing dependencies from test-requirements

Removing pep8, pyflakes and flake8 from test-requirements.
These packages were removed from global requirements.
(Commit b1101620fc0ba1938708b2e93f529341a016f0ae)

Change-Id: I2a03d6a59e2f31153695e6f45a79d56139323988
Closes-Bug: #1306256