Userland: Remove dummy IPC methods

They're not used anywhere and are unnecessary boilerplate code. So let's
remove them and update IPCCompiler to allow for empty endpoint
declarations.

Userland/Libraries/LibImageDecoderClient/Client.cpp

@@ -20,10 +20,6 @@ void Client::die()
         on_death();
 }
 
-void Client::dummy()
-{
-}
-
 Optional<DecodedImage> Client::decode_image(const ByteBuffer& encoded_data)
 {
     if (encoded_data.is_empty())

Userland/Libraries/LibImageDecoderClient/Client.h

@@ -38,8 +38,6 @@ private:
     Client();
 
     virtual void die() override;
-
-    virtual void dummy() override;
 };
 
 }