Prebuild: Remove --multi-image.

The option is ignored since
    https://android-review.googlesource.com/784718 .

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Test: Pixel 2 XL boots.
Test: testrunner.py --target --optimizing
Bug: 77856493
Change-Id: I32c323c0cabbd87cd1993c4355a354bc65529bc0