Remove /sys from file_contexts

/sys was getting labeled as a rootfs file, but according to
genfs_contexts, it's really a sysfs file. This conflict is causing
problems when patch f29c533c49ab1c90eae612b1c454f2c6879a6658 from
system/core is applied.

Change-Id: I3f34c9ee68bedb171ebebfcd356e924c987b58ff