ui: remove support for SDL1.2 in favour of SDL2

SDL1.2 was deprecated in the 2.12.0 release with:

  commit e52c6ba3
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Mon Jan 15 14:25:33 2018 +0000

    ui: deprecate use of SDL 1.2 in favour of 2.0 series

    The SDL 2.0 release was made in Aug, 2013:

      https://www.libsdl.org/release/



    That will soon be 4 + 1/2 years ago, which is enough time to consider
    the 2.0 series widely supported.

    Thus we deprecate the SDL 1.2 support, which will allow us to delete it
    in the last release of 2018. By this time, SDL 2.0 will be more than 5
    years old.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Message-id: 20180115142533.24585-1-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

It is thus able to be removed in the 3.1.0 release.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20180822131554.3398-4-berrange@redhat.com>

[ kraxel: rebase ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>