Remove superfluous(?) DCHECK.

If we don't have dex files, we have warnings telling the user:
dex2oat W 01-07 13:00:57 72639 72639 dex2oat.cc:2497] Skipping non-existent dex file 'foo.dex'

Test: test.py
Change-Id: I1b9185e5cd47f1ccbafc2ddd75d92d31b385845a
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index 345a8a0..e4aef8e 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -2535,7 +2535,6 @@
     } else {
       DCHECK_EQ(oat_writers_.size(), 1u);
       DCHECK_EQ(dex_filenames_.size(), dex_locations_.size());
-      DCHECK_NE(dex_filenames_.size(), 0u);
       for (size_t i = 0; i != dex_filenames_.size(); ++i) {
         if (!oat_writers_[0]->AddDexFileSource(dex_filenames_[i].c_str(),
                                                dex_locations_[i].c_str())) {