By Dina Kyriakidou ATHENS (Reuters) - One of Greece's most popular cartoonists has pulled down his Facebook page after what appeared to be a concerted attack by pro-government cyber bullies. A cartoon published by Arkas last month seemed anodyne enough: a newscaster saying "I have complete trust in the government. It has its feet firmly planted in the clouds." But in Greece's polarised political climate, this prompted an avalanche of hostile messages.