[modular][cleanup] Remove unused method def StoryProviderImpl.NotifyImportanceWatchers().

TEST=none

Change-Id: I1162d67c4363e0283444958e6bfc7777a7c468ef
diff --git a/bin/sessionmgr/story_runner/story_provider_impl.h b/bin/sessionmgr/story_runner/story_provider_impl.h
index ca04c50..625abbe 100644
--- a/bin/sessionmgr/story_runner/story_provider_impl.h
+++ b/bin/sessionmgr/story_runner/story_provider_impl.h
@@ -209,8 +209,6 @@
   void OnStoryStorageUpdated(fidl::StringPtr story_id,
                              fuchsia::modular::internal::StoryData story_data);
 
-  void NotifyImportanceWatchers();
-
   void NotifyStoryWatchers(
       const fuchsia::modular::internal::StoryData* const story_data,
       fuchsia::modular::StoryState story_state,