Removed SDL_GetDisplayDPI()

This function wasn't consistently correct across platforms and devices.

If you want the UI scale factor, you can use display_scale in the structure returned by SDL_GetDesktopDisplayMode(). If you need an approximate DPI, you can multiply this value times 160 on iPhone and Android, and 96 on other platforms.

android-project/app/src/main/java/org/libsdl/app/SDLActivity.java

@@ -1198,13 +1198,6 @@ public class SDLActivity extends Activity implements View.OnSystemUiVisibilityCh
         }
     }
 
-    /**
-     * This method is called by SDL using JNI.
-     */
-    public static DisplayMetrics getDisplayDPI() {
-        return getContext().getResources().getDisplayMetrics();
-    }
-
     /**
      * This method is called by SDL using JNI.
      */