remove flake8 option from run_tests.sh

flake8 will read the section [flake8] of tox.ini to guide the
behavior. In run_tests.sh, the function run_flake8() does not
need the falke8 option any more.

Fixes Bug #1214159
Change-Id: I1bbba279a339e35c776a6969598b398f57fd7646