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: 38138251
Test: m test-art-host

(cherry picked from commit 84f82b99)

Change-Id: I980f850ab0524812d18c3af34073cae25484837a