By Elizabeth Piper, Kate Holton and Alistair Smout LONDON (Reuters) -Britain’s Boris Johnson defied pressure from senior ministers and a mounting rebellion within his party to quit on Wednesday, vowing to stay on as prime minister and fight off any attempts to oust him. After more than 30 resignations from within the government and with many lawmakers in his Conservative Party in open revolt, some cabinet ministers went to Downing Street to tell Johnson he needed to go, a source said. One encouraged him to make a dignified exit by setting his own timetable rather than face a confidence vote. B…