exec: eliminate io_mem_ram

It is never used, the IOTLB always goes through io_mem_notdirty.

In fact in softmmu_template.h, if it were, QEMU would crash just
below the tests, as soon as io_mem_read/write dispatches to
error_mem_read/write.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
