NanoImage::isValid is useless now, remove it

dgl/NanoVG.hpp

@@ -42,11 +42,6 @@ public:
     */
     ~NanoImage();
 
-   /**
-      Check if this is a valid image.
-    */
-    bool isValid() const noexcept;
-
    /**
       Get size.
     */

dgl/src/NanoVG.cpp

@@ -94,11 +94,6 @@ NanoImage::~NanoImage()
         nvgDeleteImage(fContext, fImageId);
 }
 
-bool NanoImage::isValid() const noexcept
-{
-    return (fContext != nullptr && fImageId != 0);
-}
-
 Size<int> NanoImage::getSize() const
 {
     int w=0, h=0;