The macOS+iOS-specific BitmapSystemData::rImageContext was not used anywhere

Change-Id: I7190a44d4857fd337fb49e689cd71ffb78b86b9d

include/vcl/bitmap.hxx

@@ -93,7 +93,7 @@ struct BitmapSystemData
     #if defined(_WIN32)
     void* pDIB; // device independent byte buffer
     #elif defined( MACOSX ) || defined( IOS )
-    void* rImageContext;     //Image context (CGContextRef)
+    // Nothing needed, apparently
     #else
     void* aPixmap;
     #endif

vcl/quartz/salbmp.cxx

@@ -999,7 +999,6 @@ bool QuartzSalBitmap::GetSystemData( BitmapSystemData& rData )
             mxGraphicContext = mxGraphicContextNew;
         }
 
-        rData.rImageContext = static_cast<void *>(mxGraphicContext);
         rData.mnWidth = mnWidth;
         rData.mnHeight = mnHeight;
     }