tools: Fix art script to delete OAT files before running dalvikvm

This way if the compiler filter (or other flags) are changed,
re-running the art script will use the new options.

In particular it fixes 'art --profile', previously it would just
reuse the OAT file from the first profiling (jit) run.

Test: art --profile -Xcompiler-option --compiler-filter=speed-profile -cp generated/benchmarks.dex benchmarks.CaffeineLoop.java.CaffeineLoop
Test: adb shell art --32 --profile -Xcompiler-option --compiler-filter=speed-profile -cp /data/local/tmp/benchmarks.dex benchmarks.CaffeineLoop.java.CaffeineLoop
Bug: 64162479
Change-Id: Ief1784b48c78464da91e71a649a6cf4b57c51b41