download: Remove EPHY_DOWNLOAD_ACTION_AUTO

Let's always take the same action when auto-opening a file as when
opening manually, instead of allowing ephy_download_do_download_action()
to confusingly invoke itself recursively.

At first, I thought it would be good to treat automatic downloads
differently, and perform MIME type safety checking on them, but not on
downloads opened manually. But I'm not so sure anymore, since it is easy
for a server to give the file a deceptive file extension but report an
accurate MIME type, which could trick the user into opening a harmful
file (e.g. a script) if we were to stop performing MIME type safety
checks on manually-opened files. So let's continue to perform that check
in both cases. But now that EPHY_DOWNLOAD_ACTION_OPEN browses to the
file if opening fails, there's no need to have special behavior for
EPHY_DOWNLOAD_ACTION_AUTO anymore. The only difference that currently
implies is that we check the safety of the MIME type once for the
server's reported MIME type, fall back to browsing to the file if it's
unsafe, then sniff the MIME type ourselves, check safety again, and
decide to browse to. There's no point in checking the safety of the
server's reported type, since we're just going to sniff it ourselves.

So just get rid of it.

https://bugzilla.gnome.org/show_bug.cgi?id=610586