Go to export_desc_units.txt in Data folder.
Find the entry for unit Hastati.
In primary_stat (or something like that) you'll see the word "prec". Just remove that. (NOTE: if "prec" is the only entry there, i.e. no "ap", "bp" or others, you MUST replace it with a simple "no". Otherwise, just remove it).

Now your Hastati acts like a skirmisher.