"Prune" refers to something that is cleanly edged and perfect. This applies to cleaning away the rough/sharp edges of something, usually roses/bushes. In this stanza, "the pruned contours" are those things that are now perfect and lack an edge because the speaker states they all "dissolve." The connotation of "dissolve" is significant in that it is more permanent. When something dissolves, its physical composition is broken down and ceases to exist, forever. If Loy had used "disappeared" there is still a chance it can reappear again.