Hi annamack, every arch we've either built or demo'd (in a wood-framed wall) has been created in much the same way: a rectangular opening is created first (with either a load-bearing or non load-bearing header), then the arch is framed below that. We typically build them from two 3/4" plywood sizes (cut to the arch shape) with short 2X struts between them. So you'd just need to remove that along with the cornerbead and drywall from the opening.

Arches in wood-framed walls are never load-bearing, but the header above them can be. If you're simply squaring off the opening, and not widening it, the most you'll have to do other than the finish work, is to possibly furr down the header to match adjacent doorways, are they're often framed a little higher than normal.

Steve