memory: Remove old_mmio accessors

Now that all the users of old_mmio MemoryRegion accessors
have been converted, we can remove the core code support.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20180824170422.5783-2-peter.maydell@linaro.org>
Based-on: <20180802174042.29234-1-peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>