Remove deprecated -enable-hax option

Remove deprecated -enable-hax option

Our command line interface is really quite overcrowded, we should avoid
duplicated options that do the same thing in just a slightly different
way. "-accel hax" is shorter and more generic that "-enable-hax", so
there is really no real usage for the latter option. "-enable-hax" has
been deprecated since two releases, and nobody complained so far, so
it's time to remove this now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1544790073-23049-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>