ART: Clean up less in dex2oat

In non-debug builds, clean up even less. We already did not shut
down the runtime. Also skipping the compiler driver and the
verification results removes all major points of destructor
performance.

Tested with a common large app on Nexus 9. Time between dex2oat timing
message and executable exit (log from immediately-after log echo)
[w/o swap, w/ swap].
Before:  2.409s / 48.774s
After:   0.132s /  0.188s

Bug: 24199200
Change-Id: I5d8c17f8e28796545cfbb3887c07c92905f9b48d