Stop using the runtime classpath in dex2oat

Use only --class-loader-context to infer the classpath that should be used
for compilation.

The tests and benchmarks no longer rely on --runtime-arg --classpath being
passed to dex2oat.

Bug: 36044779
Test: m test-art-host
Change-Id: Iba4cb3489f4a1a4a421071e1889d806d6c1d0738