Revert "[regexp] Enable fallback to experimental engine by default"

This reverts commit 1e1f9ffc.

Reason for revert: https://bugs.chromium.org/p/chromium/issues/detail?id=1139304

Original change's description:
> [regexp] Enable fallback to experimental engine by default
>
> This CL enables the functionality that was added in d4febb6b by
> flipping the corresponding feature flag.
>
> Cq-Include-Trybots: luci.v8.try:v8_linux64_fyi_rel_ng
> Bug: v8:10765
> Bug: v8:11021
> Change-Id: Id061a274b016c71e6a4f7d7934a9c287d3124228
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2470568
> Commit-Queue: Martin Bidlingmaier <mbid@google.com>
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#70510}

TBR=jgruber@chromium.org,mbid@google.com

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:10765,v8:11021,chromium:1139304
Change-Id: Ie9c38cc30d1fa81d395cc11ab1f1db5e19ba04de
Cq-Include-Trybots: luci.v8.try:v8_linux64_fyi_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2484402
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70594}

src/flags/flag-definitions.h

@@ -1493,7 +1493,7 @@ DEFINE_BOOL(enable_experimental_regexp_engine, false,
 DEFINE_BOOL(trace_experimental_regexp_engine, false,
             "trace execution of experimental regexp engine")
 
-DEFINE_BOOL(enable_experimental_regexp_engine_on_excessive_backtracks, true,
+DEFINE_BOOL(enable_experimental_regexp_engine_on_excessive_backtracks, false,
             "fall back to a breadth-first regexp engine on excessive "
             "backtracking")
 DEFINE_UINT(regexp_backtracks_before_fallback, 10000,