Revert "exec: limit system memory size"

This reverts commit 818f86b88394b7b2b59d313e51043fe15a8004db.

This was a work-around for bugs elsewhere in the system,
exposed by commit a53ae8e934cd54686875b5bcfc2f434244ee55d6:
    "hw/pci: partially handle pci master abort"
since that's reverted now, the work-around is not required for 1.7
anymore.
The proper fix is supporting full 64 bit addresses in the radix tree.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Marcel Apfelbaum <marcel.a@redhat.com>
