remove unused LinkManager::removeLink

remove unused LinkManager::removeLink which limits the way in which LinkManager can implement its payload of links (which currently are stable under both addition and removal of links, which implies owned heap storage). By removing removeLink it becomes possible to eg. store the links by value in an std::deque. In addition, links are indexed by position, and thus removing links changes the indices of subsequent links, information which potentially has already been used -- as a result calling removeLink could have unforeseen side-effects. Hence, better to just remove removeLinks.