bug: enabling pruning from initial launch causes crash due to "txindex"

[PastaPastaPasta]

Reproduce:

1. Clear out ~/.dashcore
2. run dash-qt
3. select "prune" option
4. Observe crash
   "Error: Prune mode is incompatible with -txindex"

This should be as simple as setting "txindex" off when the gui option is set. My guess is that prune is set too late for the Init params to override txindex, but then the compatibility check happens later, so we crash? Haven't actually looked at code related to this.

Initially observed here: flathub/flathub#5747 (comment)

Thanks @bbhtt!