Remove unneeded --retry-crashes flag from run-webkit-tests.

We now always retry crashes; the flag was still there only for compatibility
with bots that no longer need it.

R=ojan@chromium.org
BUG=269204

Review URL: https://chromiumcodereview.appspot.com/22907005

git-svn-id: svn://svn.chromium.org/blink/trunk@156061 bbb929c8-8fbe-4397-9dbb-9b2b20218538