update_engine: Remove xattr checks

It seems like the builders support xattr, otherwise some unittests like
P2PDownloadActionTest should've been failed. Although, this is in conterary with
crbug.com/817524. Since they haven't failed it means we can remove the checks
for xattrs support in some of the unittests.

BUG=none
TEST=precq
TEST=unittests

Change-Id: I81fa96d21ae06ee2f832c1c3063fc78229238b35
Reviewed-on: https://chromium-review.googlesource.com/1171189


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>