This reverts commit 7a0d6a50.

Reason for revert: Breaks no-slow-assert builds.

Original change's description:
> [build] Disable slow asserts for v8_enable_fast_mksnapshot
> 
> Slow asserts double the time spent in mksnapshot. While we want them
> enabled on our bot builds, local builds should be fast and don't
> necessarily need them.
> 
> This also adds v8_enable_fast_mksnapshot as default in gm.py.
> 
> Bug: v8:7891,v8:6688
> Change-Id: I5ab1491f3396c4351de30dbbb80a81e2242c5db9
> Reviewed-on: https://chromium-review.googlesource.com/1116552
> Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
> Commit-Queue: Jakob Gruber <jgruber@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54074}

TBR=jkummerow@chromium.org,jgruber@chromium.org

Change-Id: I6b048e819846c4bd9a064f82e2f3c7609369cd33
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7891, v8:6688
Reviewed-on: https://chromium-review.googlesource.com/1118138Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54077}