target-i386: Remove unused X86CPUDefinition::xlevel2 field

No CPU model in builtin_x86_defs has xlevel2 set, so it is always
zero. Delete the field.

Note that this is not an user-visible change. It doesn't remove
the ability to set xlevel2 on the command-line, it just removes
an unused field in builtin_x86_defs.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>