When she deleted the wife she deleted her from the entire program. Sullivan has never been married and no one in the entire village has known this wife. She has never existed for them. So it's not as if she bloodlessly murdered her so she can be a post-death mistress free from an avenging wife.
She rewrote Sullivan as single.
However had it gone otherwise it would have just been an interesting glimpse into Janeway's fantasy life, not cheating as these are just play things not people. While you could possibly make a case for holosex being cheating on your real life
partner (in the same way some would consider porn to be cheating) it's not real cheating for a single person to have sex with a hologram who has a hologram partner.
As it is we didn't get any of that. Janeway is a fairly traditional prosaic person and the poetry reading courtship is much more her style than playing the seducing mistress.