Update transport.md

Removing paragraph on 'train automation' – this technology doesn't really exist outside of metro systems, partly because it wouldn't have a huge benefit on longer-distance routes (also because of higher speeds). Far more important to press ahead with electrification and signalling upgrades (which automatically stops the train from passing red signals).

London Underground is a devolved issue (to the GLA), and so shouldn't be discussed in this manifesto IMO.