This reverts commit 6366a010.

Reason for revert: Breaks layout tests:
https://build.chromium.org/p/client.v8.fyi/builders/V8-Blink%20Linux%2064/builds/19429

Original change's description:
> [ic] Internalize strings on the fly in KeyedLoadICGeneric.
> 
> This turns on the existing --internalize_on_the_fly flag for the
> MEGAMORPHIC KeyedLoadIC to properly internalize strings before
> looking up the property. This avoids the otherwise taken runtime
> call to %KeyedGetProperty, which is definitely slower.
> 
> Initially the --internalize_on_the_fly flag was turned off because
> internalizing strings on the fly causes too much traffic on the
> megamorphic stub cache. We avoid this problem here by not probing
> the stub cache in that case, which still gives the benefit of not
> having to go to the runtime.
> 
> This improves the babylon test on the web-tooling-benchmark by around
> 2-3% and will probably also help with several tests (like React or
> Ember) on the Speedometer benchmark.
> 
> If this CL causes trouble (i.e. tanks something important), we can
> just turn off the --internalize_on_the_fly flag again.
> 
> Bug: v8:6936, v8:7026
> Change-Id: Ia59a8a3799d9624d831d66b05bae3ecef31cee0a
> Reviewed-on: https://chromium-review.googlesource.com/750821
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49072}

TBR=ishell@chromium.org,bmeurer@chromium.org

Change-Id: I5345eb29016ecd6b7788b1b49b2f53992ea82b58
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6936, v8:7026
Reviewed-on: https://chromium-review.googlesource.com/750904Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49077}