block/curl: Drop TFTP "support"

Because TFTP does not support byte ranges, it was never usable with our
curl block driver. Since apparently nobody has ever complained loudly
enough for someone to take care of the issue until now, it seems
reasonable to assume that nobody has ever actually used it.

Therefore, it should be safe to just drop it from curl's protocol list.

[Jeff Cody: Below is additional summary pulled, with some rewording,
            from followup emails between Max and Markus, to explain what
            worked and what didn't]

TFTP would sometimes work, to a limited extent, for images <= the curl
"readahead" size, so long as reads started at offset zero.  By default,
that readahead size is 256KB.

Reads starting at a non-zero offset would also have returned data from a
zero offset.  It can become more complicated still, with mixed reads at
zero offset and non-zero offsets, due to data buffering.

In short, TFTP could only have worked before in very specific scenarios
with unrealistic expectations and constraints.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 20161102175539.4375-4-mreitz@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>