[DevTools] Remove promise inspector experiment

We decided to remove this experiment because it doesn't really help in most scenarios. We should instead polish async stacks and finish async stepping to make debugging of promises and any other async code easy.

BUG=526811,348919,587850
R=dgozman@chromium.org,pfeldman@chromium.org

Review URL: https://codereview.chromium.org/1827993002

Cr-Commit-Position: refs/heads/master@{#383328}
26 files changed