memory: Remove code for mr->may_overlap

The collision check does nothing and hasn't been used. Remove the
variable together with related code.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1458900629-2334-2-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
