Don't show captured units in summary.

Since siege isn't capturable anymore since r24685 / rP24685.

Reported by: fmmaks at https://wildfiregames.com/forum/topic/73490-cant-capture-siege-engines-alpha-25/#comment-484544.
Patch by: @Langbart
Differential revision: https://code.wildfiregames.com/D4538
Fixes #6455