Don't label /dev/cam from system sepolicy

With Treble, cameraserver no longer depends on camera devices directly.
Moreover, pixel 3 doesn't have /dev/cam node.

We still keep "camera_device" type around since vendor policy uses it to
label its /dev nodes.

Bug: 110962171
Test: boot aosp_walleye
Test: camera app still works
Change-Id: If12d640c2a0006b9fc3c9f6704285eb8eb66c626