Drop the unused prefix parameter in do_drop_stash.
We also have an unused "prefix" parameter in the 'create_stash'
function, however we leave that in place for symmetry with the other
top-level functions.
Reported-by: Jeff King <peff@peff.net> Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>