Remove dmesg_restrict

dmesg_restrict is too coarse of a control. In Android's case,
we want to allow the shell user to see dmesg output, but disallow
others from seeing it.

Rather than rely on dmesg_restrict, use SELinux to control access
to dmesg instead. See corresponding change in external/sepolicy .

Bug: 10020939
Change-Id: I9d4bbbd41cb02b707cdfee79f826a39c1ec2f177