ui/sdl_keysym: Remove obsolete SDL1.2 related code

sdl_keysym.h has only been included by sdl.c which has recently been
removed recently with this commit:

  0015ca5
  ("ui: remove support for SDL1.2 in favour of SDL2")

So we can drop this header file now completely, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 1549282241-23535-1-git-send-email-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>