DevTools: cleanup debugger tests.

Debugger is always enabled at the moment when we run the test so we don't need to check that and dump the message.
Also we don't need to disable it manually at the end because it might hide an error as it happens with debugger-step-into-custom-element-callbacks.html

BUG=

Review URL: https://codereview.chromium.org/316403002

git-svn-id: svn://svn.chromium.org/blink/trunk@175870 bbb929c8-8fbe-4397-9dbb-9b2b20218538