powerpc/pseries: drop RTAS-based timebase synchronization

The pseries platform has been LPAR-only for several generations, and
the PAPR spec:

* Guarantees that timebase synchronization is performed by
  the platform ("The timebase registers are synchronized by the
  platform before CPUs are given to the OS" - 7.3.8 SMP Support).

* Completely omits the RTAS freeze-time-base and thaw-time-base RTAS
  functions, which are CHRP artifacts.

This code is effectively unused on currently supported models, so drop
it.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230125-b4-powerpc-rtas-queue-v3-7-26929c8cce78@linux.ibm.com