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
1 file changed