removing assertion about current thread in next_fragment

next_fragment is called outside the scope of a high_priority_switch,
and we may be in a different thread context then the streaming applier
was created in

include/wsrep/client_state.hpp

@@ -439,7 +439,6 @@ namespace wsrep
         int next_fragment(const wsrep::ws_meta& meta)
         {
             wsrep::unique_lock<wsrep::mutex> lock(mutex_);
-            assert(current_thread_id_ == wsrep::this_thread::get_id());
             assert(mode_ == m_high_priority);
             return transaction_.next_fragment(meta);
         }