Do not restore source object hash in ImageWriter.

Writing the image is the last step, so these values shall
not be needed anymore. And if we decided to restore the heap
to a usable state, we would have to clear the lockword of
all other objects as well.

Test: m test-art-host-gtest
Test: Pixel 2 XL boots.
Change-Id: I6cc807808b49f89e20f0141bcc2ad20b9005a614