Bug 122854 There is redundant columns in "available columns" list in "export data" dialog.
Description: There is redundant columns in "available columns" list in "export data" dialog. Steps to reproduce: 1. Preview attached design in web viewer. 2. Select the table and click on "export" button. 3. There is a "detail" column name in "available columns" list which is not data set column.
engine/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/ir/Report.java
*            content index
	 * @return content
	 */
	public ReportItemDesign getContent( int index )
	{
		assert ( index >= 0 && index < this.contents.size( ) );
		return (ReportItemDesign) this.contents.get( index );
	}
	/**
	 * add content in to report body.
	 * 
	 * @param item
	 *            content to be added.
	 */
	public void addContent( ReportItemDesign item )
	{
		this.contents.add( item );
	}
	/**
	 * get number of shared styles defined in this report.
	 * 
	 * @return style number
	 */
	public int getStyleCount( )
	{
		return this.styles.size( );
	}
	/**
	 * get the style.
	 *