hw/misc: Remove mmio_interface device

hw/misc: Remove mmio_interface device

The mmio_interface device was a purely internal artifact
of the implementation of the memory subsystem's request_ptr
APIs. Now that we have removed those APIs, we can remove
the mmio_interface device too.

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: 20180817114619.22354-4-peter.maydell@linaro.org