LONDON (Reuters) - Analysts on Friday slammed Royal Dutch Shell's warning of possible delays to its $25 billion (£19 billion) share buyback as an unnecessary step that undermines the energy giant's management. Shell, the world's second-largest listed oil and gas company, saw its shares close more than 4% lower on Thursday, wiping out $10 billion of its market value. It had earlier reported stronger-than-expected third-quarter profits which were, however, overshadowed by Chief Executive Ben van..