Remove dead labs flag feature_html_topic (#34103)

It was delabsed in https://github.com/element-hq/element-web/pull/29817

apps/web/src/i18n/strings/en_EN.json

@@ -1550,7 +1550,6 @@
         "group_voip": "Voice & Video",
         "group_widgets": "Widgets",
         "hidebold": "Hide notification dot (only display counters badges)",
-        "html_topic": "Show HTML representation of room topics",
         "join_beta": "Join the beta",
         "join_beta_reload": "Joining the beta will reload %(brand)s.",
         "jump_to_date": "Jump to date (adds /jumptodate and jump to date headers)",

apps/web/src/settings/Settings.tsx

@@ -214,7 +214,6 @@ export interface Settings {
     "feature_mjolnir": IFeature;
     "feature_custom_themes": IFeature;
     "feature_exclude_insecure_devices": IFeature;
-    "feature_html_topic": IFeature;
     "feature_bridge_state": IFeature;
     "feature_jump_to_date": IFeature;
     "feature_sliding_sync": IBaseSetting<boolean>;
@@ -524,14 +523,6 @@ export const SETTINGS: Settings = {
         // Contains room ID
         shouldExportToRageshake: false,
     },
-    "feature_html_topic": {
-        isFeature: true,
-        labsGroup: LabGroup.Rooms,
-        supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS_WITH_CONFIG_PRIORITISED,
-        supportedLevelsAreOrdered: true,
-        displayName: _td("labs|html_topic"),
-        default: false,
-    },
     "feature_bridge_state": {
         isFeature: true,
         labsGroup: LabGroup.Rooms,