Don't hold on to the background print color option

When printing b/w we need to set the print background to off otherwise
we can inadvertently get backgrounds printing

eeschema/dialogs/dialog_print_using_printer.cpp

@@ -250,6 +250,8 @@ void DIALOG_PRINT_USING_PRINTER::SavePrintOptions()
 
     if( m_checkBackgroundColor->IsEnabled() )
         cfg->m_Printing.background = m_checkBackgroundColor->IsChecked();
+    else
+        cfg->m_Printing.background = false;
 
     cfg->m_Printing.use_theme   = m_checkUseColorTheme->IsChecked();