LSIF: Remove cors middleware.
Created by: efritz
CORS was added to the lsif-server backend to support swagger-ui, but this isn't a hard requirements.
Until we stop proxying, we'll remove the CORS middleware here, as it causes the frontend to return two sets of header values, which messes up downstream clients.
See https://gitlab.com/gitlab-com/gl-infra/production/issues/1385#note_248611373 for details.