While this effect was clearly produced by instinctive magic, it seems most similar to the vanishing spell Evanesco, which by its inclusion in Professor McGonnagall's class would presumably be counted as a transfiguration. Indeed, in Order of the Phoenix, chapter 13 "Detention with Dolores" Professor McGonagall sets her fifth year students on a task to practicing vanishing spells on snails, categorising them as "among the most difficult magic [the students] will be tested on in [their] O.W.L".
And to answer the question in the comments as to where vanished objects go:
"Into non-being, which is to say, everything"
- Professor McGonagall to the Ravenclaw door knocker, Harry Potter and the Deathly Hallows