vl: drop needless -spice checks

Since commit 5324e3e9 ("qemu-options: define -spice only #ifdef
CONFIG_SPICE"), it is unnecessary to check at runtime for "-spice"
option.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>