Simplify genfs labeling of tracefs.

The code used to look like this, but in commit
4cae28d4 we replaced the generic
regexes to improve performance.  Now that we've switched to genfs,
this no longer affects performance, so let's simplify the labeling.

Bug: 62413700
Test: Built, flashed, and booted two devices.  Verified that all of
the files have the correct context and that wifi, camera, and traceur
work.

Change-Id: I1a859d17075fa25543ee090cc7a7478391bc45c1