Fix BroadcasterManager::RemoveListener to really remove the listener

This appears to be a real bug caught by -Wunused-value. std::find_if
doesn't modify the underlying collection, it just returns an iterator
pointing to the matching element.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D74010

Author: rnk

lldb/source/Utility/Broadcaster.cpp

@@ -406,7 +406,7 @@ void BroadcasterManager::RemoveListener(Listener *listener) {
   listener_collection::iterator iter = m_listeners.begin(),
                                 end_iter = m_listeners.end();
 
-  std::find_if(iter, end_iter, predicate);
+  iter = std::find_if(iter, end_iter, predicate);
   if (iter != end_iter)
     m_listeners.erase(iter);