Remove note about auras from buildings having standard aura descriptions

[Krinkle]

Looking at iber_monument for example, in r17974 (ref #3792) the logic for auras was factored out of the structure template XML files, and into dedicated JSON files.

It also added the ability for the aura component to expose its own dedicated tooltip section.

This was then made use of for iber_monument in r18125 (ref #3947), but the old note about "units fighting harder" in the template XML "<Tooltip>" was left behind, which is now presenting the same information twice in the same tooltip.

[Krinkle]

From ​https://code.wildfiregames.com/D2005#83640

the tooltips of iber_monument.xml, maur_pillar_ashoka.xml, and pers_ishtar_gate.xml can be purged.