arg : clean up handling --mmproj with -hf (#13082)

* arg : clean up handling --mmproj with -hf

* rm change about no_mmproj

* Revert "rm change about no_mmproj"

This reverts commit 2cac8e0efb.

* handle no_mmproj explicitly

* skip download mmproj on examples not using it

common/common.h

@@ -342,6 +342,7 @@ struct common_params {
 
     // multimodal models (see examples/llava)
     struct common_params_model mmproj;
+    bool no_mmproj = false;         // explicitly disable multimodal model
     std::vector<std::string> image; // path to image file(s)
 
     // embedding