KVM: x86: Drop RF manipulation for guest single-stepping

RF is not required for injecting TF as the latter will trigger only
after an instruction execution anyway. So do not touch RF when arming or
disarming guest single-step mode.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>