remove flake8 option in run_tests.sh

flake8 read [flake8] section of tox.ini as options.

function run_flake8 in run_tests.sh should be kept simple,
and tox.ini controls the behavior of flake8.

Fixes Bug #1212988

Change-Id: I35d434ccea1ca2f3ff93657e4ca6dde93bbbf29e
Merge "remove flake8 option in run_tests.sh"