Remove unused colorIndexed member.

As all the indexed colors will be converted to rgb in the library when
reads an existing .xlsx file.

When write a .xlsx file, we never write indexed color.

src/xlsx/xlsxformat.h

@@ -253,7 +253,6 @@ private:
     void setDxfIndex(int index);
 
     int theme() const;
-    int colorIndexed() const;
 
     FormatPrivate * const d_ptr;
 };