wizard: Remove redundant & problematic check

For some reason we have been bailing out on URLs if their mime-type
could be detected with 100% certainty. While I don't recall exactly why
this was added back in the days, I'm pretty sure its not needed right
now. At the very least, this check fails when Boxes is given URL of
remote non-ISO (import) images.

https://bugzilla.gnome.org/show_bug.cgi?id=737113