hw/ssi/xilinx_spips: Remove unneeded MMIO request_ptr code

hw/ssi/xilinx_spips: Remove unneeded MMIO request_ptr code

We now support direct execution from MMIO regions in the
core memory subsystem. This means that we don't need to
have device-specific support for it, and we can remove
the request_ptr handling from the Xilinx SPIPS device.
(It was broken anyway due to race conditions, and disabled
by default.)

This device is the only in-tree user of this API.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: KONRAD Frederic <frederic.konrad@adacore.com>
Message-id: 20180817114619.22354-2-peter.maydell@linaro.org