Removed TEST_COOKIE_NAME and TEST_COOKIE_VALUE constants from

sessions.middleware. They haven't been used here since the great refactor and
are duplicated in sessions.backends.base.SessionBase.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8156 bcc190cf-cafb-0310-a4f2-bffc1f526a37