x86/pv: Remove deferred RDTSC{,P} handling in pv_emulate_privileged_op()

x86/pv: Remove deferred RDTSC{,P} handling in pv_emulate_privileged_op()

As noted in c/s 4999bf3e8b "x86/PV: use generic emulator for privileged
instruction handling", these hoops are jumped through to retain the older
behaviour, along with a note suggesting that we should reconsider things.

Part of the reason for retention of the old behaviour was removed by c/s
5b04262079 "x86/time: Rework pv_soft_rdtsc() to aid further cleanup" which in
particular caused it to not write regs->rcx directly.

It does not matter exactly when pv_soft_rdtsc() is called, as Xen's behaviour
is an opaque atomic action from the guests point of view.

Drop all the deferral logic, and leave TSC_AUX uniformly at 0 as PVRDTSCP mode
is being removed.  Later changes will make this behave architecturally.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>