r/persisted_stm: fixed skipping application of raft snapshot

When `persisted_stm` starts it must apply both local and raft snapshot.

When Raft snapshot is present or local snapshot is out of date it must
be applied to the stm. Removed code that was setting next offset
preventing Raft snapshot from being applied.

Fixes: redpanda-data#15127

Signed-off-by: Michal Maslanka <michal@redpanda.com>
(cherry picked from commit c9c675e)

src/v/cluster/persisted_stm.cc

@@ -564,23 +564,11 @@ ss::future<> persisted_stm<T>::start() {
               _log.warn,
               "Skipping snapshot {} since it's out of sync with the log",
               _snapshot_backend.store_path());
-            vlog(
-              _log.debug,
-              "start with non-applied snapshot, set_next {}",
-              next_offset);
-            _insync_offset = model::prev_offset(next_offset);
-            set_next(next_offset);
         }
 
         _resolved_when_snapshot_hydrated.set_value();
     } else {
-        auto offset = _c->start_offset();
-        vlog(_log.debug, "start without snapshot, maybe set_next {}", offset);
-
-        if (offset >= model::offset(0)) {
-            _insync_offset = model::prev_offset(offset);
-            set_next(offset);
-        }
+        vlog(_log.debug, "starting without snapshot");
         _resolved_when_snapshot_hydrated.set_value();
     }
     co_await state_machine::start();