Remove additionalSettings

Per opensearch-project/OpenSearch#9392, mmap will be
a default setting for all lucene segment files

Signed-off-by: Heemin Kim <heemin@amazon.com>

src/main/java/org/opensearch/knn/plugin/KNNPlugin.java

@@ -354,25 +354,4 @@ public List<NamedXContentRegistry.Entry> getNamedXContent() {
     public Collection<SystemIndexDescriptor> getSystemIndexDescriptors(Settings settings) {
         return ImmutableList.of(new SystemIndexDescriptor(MODEL_INDEX_NAME, "Index for storing models used for k-NN indices"));
     }
-
-    /**
-     * Plugin can provide additional node settings, that includes new settings or overrides for existing one from core.
-     *
-     * @return settings that are set by plugin
-     */
-    @Override
-    public Settings additionalSettings() {
-        // We add engine specific extensions to the core list for HybridFS store type. We read existing values
-        // and append ours because in core setting will be replaced by override.
-        // Values are set as cluster defaults and are used at index creation time. Index specific overrides will take priority over values
-        // that are set here.
-        final List<String> engineSettings = Arrays.stream(KNNEngine.values())
-            .flatMap(engine -> engine.mmapFileExtensions().stream())
-            .collect(Collectors.toList());
-        final List<String> combinedSettings = Stream.concat(
-            IndexModule.INDEX_STORE_HYBRID_MMAP_EXTENSIONS.getDefault(Settings.EMPTY).stream(),
-            engineSettings.stream()
-        ).collect(Collectors.toList());
-        return Settings.builder().putList(IndexModule.INDEX_STORE_HYBRID_MMAP_EXTENSIONS.getKey(), combinedSettings).build();
-    }
 }