Remove the obsolete MMIO request_ptr APIs; they have no
users now.

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: 20180817114619.22354-3-peter.maydell@linaro.org