Delete DexFile closeguard

Since class unloading deletes class loaders, it can cause CloseGuard
warnings for the dex files in the dex path list. The parent finalizer
can not call close() since the finalizer ordering is not specified.

Test: test-art-host

Bug: 29820420

(cherry picked from commit 97db693d)

Change-Id: I3d5bb133aa2ae537fb1731f99dbd1723811260c2