Revert "[regexp] Stage regexp match indices."

This reverts commit fe7b8b80.

Reason for revert: Clusterfuzz has exposed a number of issues.

Bug: chromium:1013135, chromium:1013013, chromium:1013058, chromium:1013133

Original change's description:
> [regexp] Stage regexp match indices.
> 
> Bug: v8:9548
> Change-Id: I75fdb71154a728f34c5a5b02ae9d75f607b20c69
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1849838
> Reviewed-by: Adam Klein <adamk@chromium.org>
> Commit-Queue: Joshua Litt <joshualitt@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#64204}

TBR=adamk@chromium.org,joshualitt@chromium.org

Change-Id: Icab215601d31a25ece9d21807285ca290ea21f9d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:9548
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1852194Reviewed-by: Joshua Litt <joshualitt@chromium.org>
Commit-Queue: Joshua Litt <joshualitt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64222}

src/flags/flag-definitions.h

@@ -207,6 +207,7 @@ DEFINE_IMPLICATION(harmony_import_meta, harmony_dynamic_import)
   V(harmony_private_methods, "harmony private methods in class literals") \
   V(harmony_regexp_sequence, "RegExp Unicode sequence properties")        \
   V(harmony_weak_refs, "harmony weak references")                         \
+  V(harmony_regexp_match_indices, "harmony regexp match indices")         \
   V(harmony_nullish, "harmony nullish operator")                          \
   V(harmony_top_level_await, "harmony top level await")
 
@@ -218,8 +219,7 @@ DEFINE_IMPLICATION(harmony_import_meta, harmony_dynamic_import)
 
 // Features that are complete (but still behind --harmony/es-staging flag).
 #define HARMONY_STAGED_BASE(V)                                     \
-  V(harmony_optional_chaining, "harmony optional chaining syntax") \
-  V(harmony_regexp_match_indices, "harmony regexp match indices")
+  V(harmony_optional_chaining, "harmony optional chaining syntax")
 
 #ifdef V8_INTL_SUPPORT
 #define HARMONY_STAGED(V)                                           \