Revert of [esnext] ship String.prototype.padStart / String.prototype.padEnd...

Revert of [esnext] ship String.prototype.padStart / String.prototype.padEnd (patchset #1 id:1 of https://codereview.chromium.org/2382193002/ )

Reason for revert:
Causes several GC bugs, e.g. https://build.chromium.org/p/client.v8.ports/builders/V8%20Linux%20-%20arm64%20-%20sim%20-%20gc%20stress/builds/2858/steps/Mjsunit/logs/debug-constructed-by

Original issue's description:
> [esnext] ship String.prototype.padStart / String.prototype.padEnd
>
> Enable the --harmony-string-padding flag by default
>
> BUG=v8:4954
> R=adamk@chromium.org, littledan@chromium.org
>
> Committed: https://crrev.com/8352a0feaccfd9a19f3b38564ed2c0859dd6e3d5
> Cr-Commit-Position: refs/heads/master@{#40060}

TBR=littledan@chromium.org,adamk@chromium.org,foolip@chromium.org,caitp@igalia.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4954

Review-Url: https://codereview.chromium.org/2398183003
Cr-Commit-Position: refs/heads/master@{#40071}

src/flag-definitions.h

@@ -209,9 +209,10 @@ DEFINE_IMPLICATION(es_staging, move_object_start)
   V(harmony_class_fields, "harmony public fields in class literals")
 
 // Features that are complete (but still behind --harmony/es-staging flag).
-#define HARMONY_STAGED_BASE(V)                              \
-  V(harmony_regexp_lookbehind, "harmony regexp lookbehind") \
-  V(harmony_tailcalls, "harmony tail calls")
+#define HARMONY_STAGED_BASE(V)                                               \
+  V(harmony_regexp_lookbehind, "harmony regexp lookbehind")                  \
+  V(harmony_tailcalls, "harmony tail calls")                                 \
+  V(harmony_string_padding, "harmony String-padding methods")
 
 #ifdef V8_I18N_SUPPORT
 #define HARMONY_STAGED(V)                                          \
@@ -229,8 +230,7 @@ DEFINE_IMPLICATION(es_staging, move_object_start)
     "harmony limitations on sloppy mode function declarations")              \
   V(harmony_object_values_entries, "harmony Object.values / Object.entries") \
   V(harmony_object_own_property_descriptors,                                 \
-    "harmony Object.getOwnPropertyDescriptors()")                            \
-  V(harmony_string_padding, "harmony String-padding methods")
+    "harmony Object.getOwnPropertyDescriptors()")
 
 // Once a shipping feature has proved stable in the wild, it will be dropped
 // from HARMONY_SHIPPING, all occurrences of the FLAG_ variable are removed,