kmem_device was used to label /dev/mem and /dev/kmem. We already have
multiple layers of protection against those /dev nodes being present on
devices.

CTS checks that /dev/mem and /dev/kmem don't exist:
https://android.googlesource.com/platform/cts/+/master/tests/tests/permission/src/android/permission/cts/FileSystemPermissionTest.java#233

VTS enforces our base kernel configs, which have CONFIG_DEVKMEM and
CONFIG_DEVMEM disabled:
https://android.googlesource.com/kernel/configs/+/master/android-4.9/android-base.config#2

Bug: 110962171
Test: m selinux_policy
Change-Id: I246740684218dee0cddf81dabf84d4763a753cde