Drop unnecessary rdtsc_barrier(), as has been determined empirically,
see 057e6a8c for details.

Noticed by Andy Lutomirski.

Improves clock_gettime() by approximately 15% on
Intel i7-3520M @ 2.90GHz.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>