Kernel: Remove declarations for non-existent methods

Author: https://github.com/AtkinsSJ
Commit: 3cbc0fdbb0
Pull-request: https://github.com/SerenityOS/serenity/pull/17203
Reviewed-by: https://github.com/linusg ✅

Kernel/Net/Realtek/RTL8168NetworkAdapter.h

@@ -132,7 +132,6 @@ private:
     void read_mac_address();
     void set_phy_speed();
     void start_hardware();
-    void initialize();
     void startup();
 
     void configure_phy();