yes, you can definitely simplify it. I’ve include a picture to help illustrate (it’s for a pair of pants but just ignore that since it’s essentially the same.) The current boxer pattern has the beginning of a crotch curve and then the pattern line jets out for the fly section of the shorts. If you take a look at my pant example, you’ll see the crotch curve and then it goes straight up to the waistline. This is what it looks like for something without a fly and what you need to do to alter the pattern so you can sew a seam in this section instead of a fly. Hope that helps!