LONDON — U.S. software giant Microsoft has walked away from its £1.58 billion ($2.6 billion) investment in U.K. cable operator Telewest, selling its 23.6% shareholding.U.S. telco IDT, in which John Malone’s Liberty Media has a minority stake, bought Microsoft’s shares for just $5 million. Malone owns 25.2% of Telewest and controls 11% of its bonds. Microsoft, which was Telewest’s second largest shareholder, bought into cable companies with the aim of establishing a new platform for its software. So far it has written off $7 billion in cable investments since 2001. Telewest is in the final throws of completing its debt for equity swap that will cut its debt from $8.7 billion to $2.9 billion.