raft: remove empty() from fsm_output

Nobody remembered to keep this function up to date when adding stuff to
`fsm_output`.

Turns out that it's not being used by any Raft logic but only in some
tests. That use case can now be replaced with `fsm::has_output()` which
is also being used by `raft::server` code.

raft/fsm.hh

@@ -46,14 +46,6 @@ struct fsm_output {
     bool state_changed = false;
     // Set to true if a leadership transfer was aborted since the last output
     bool abort_leadership_transfer;
-
-    // True if there is no new output
-    bool empty() const {
-        return !term_and_vote &&
-            log_entries.size() == 0 && messages.size() == 0 &&
-            committed.size() == 0 && !snp && snps_to_drop.empty() &&
-            !configuration && !max_read_id_with_quorum;
-    }
 };
 
 struct fsm_config {

test/raft/helpers.cc

@@ -94,7 +94,8 @@ communicate_impl(std::function<bool()> stop_pred, raft_routing_map& map) {
         has_traffic = false;
         for (auto e : map) {
             raft::fsm& from = *e.second;
-            for (auto output = from.get_output(); !output.empty(); output = from.get_output()) {
+            for (bool has_output = from.has_output(); has_output; has_output = from.has_output()) {
+                auto output = from.get_output();
                 if (stop_pred()) {
                     return;
                 }