Clean up .coverage left around by test-webkitpy

test-webkitpy tests running the -c flag for coverage; this would cause
us to leave a .coverage file in Tools/Scripts. This patch cleans that up

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/399193006

git-svn-id: svn://svn.chromium.org/blink/trunk@178483 bbb929c8-8fbe-4397-9dbb-9b2b20218538