campaignd: Don't use GUI1 markup in [upload] response

Wesnoth 1.7.x and later use a GUI2 code path to display the [upload]
response message, which means GUI1 markup won't work with it. They do
not enable Pango markup either, and in any case, formatting messages
properly should be the client's responsibility, not the server's.

Perhaps later we should allow including a secondary message in the
response or something like that.