exec: Remove unused global variable phys_ram_fd

It seems to be unused since several years (commit
be995c27 in 2006).

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Message-id: 1373044036-14443-1-git-send-email-sw@weilnetz.de
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>