qxl: drop mono cursor support

The chunk size sanity check in qxl_render_cursor works for
SPICE_CURSOR_TYPE_ALPHA cursors only.  So support for
SPICE_CURSOR_TYPE_MONO cursors must be broken for ages without anyone
noticing.  Most likely it simply isn't used any more by guest drivers.
Drop the dead code.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170828123933.30323-2-kraxel@redhat.com
