remove unused function

src/background/main.background.ts

@@ -356,14 +356,6 @@ export default class MainBackground {
         }
     }
 
-    sendInternalRuntimeMessage(message: any) {
-        if (!this.isSafari) {
-            throw new Error('Only safari can send internal runtime messages.');
-        }
-
-        this.runtimeBackground.processMessage(message, { tab: null }, () => { /* No response needed. */ });
-    }
-
     async openPopup() {
         // Chrome APIs cannot open popup