As the title says. I've noticed during reproducing of #18921, that
after in `PagePrinter::renderPage` we call `setTemplateMarkers`
twice, which results in deleting `childItems` allocated memory
(in `setTemplateMarkers`->`setMarkers`->`updateView`->`clearClickHandles`),
and then we are calling `setTemplateMarkers` (also in `PagePrinter::renderPage`) second time,
accessing `textFields` in `setMarkers` method, which still contain hanging pointers
from the previous deallocation. This results in segfaults as we iterate through `textFields`.
So, instead of keeping sychronization between childs of QGISVGTemplate
and textFields vector - this patch removes this variable at all and uses
childs directly to return text fields.
