ui: Remove deprecated options "-sdl" and "-curses"

We have "-sdl" and "-curses", but no "-gtk" and no "-cocoa" ...
these old-style options are rather confusing than helpful nowadays.
Now that the deprecation period is over, let's remove them, so we
get a cleaner interface (where "-display" is the only way to select
the user interface).

Message-Id: <20220519155625.1414365-4-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>