Revert "Stage --harmony-function-tostring"

This reverts commit fa314341.

Reason for revert: Causes a significant bug: https://bugs.chromium.org/p/v8/issues/detail?id=6190

Original change's description:
> Stage --harmony-function-tostring
> 
> BUG=v8:4958
> 
> Change-Id: Id02d36fce76eed54a5a3d348dbac2ea7d43f4ef3
> Reviewed-on: https://chromium-review.googlesource.com/462336
> Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
> Commit-Queue: Adam Klein <adamk@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#44275}

TBR=adamk@chromium.org,littledan@chromium.org,hablich@chromium.org,v8-reviews@googlegroups.com
# Not skipping CQ checks because original CL landed > 1 day ago.
BUG=v8:4958

Change-Id: I43388674e454275fb93a15b9af03e3d8c3cfaaa2
Reviewed-on: https://chromium-review.googlesource.com/465810Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Commit-Queue: Daniel Ehrenberg <littledan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44330}

src/flag-definitions.h

@@ -207,6 +207,7 @@ DEFINE_IMPLICATION(es_staging, move_object_start)
   V(harmony_regexp_dotall, "harmony regexp dotall flag")                \
   V(harmony_regexp_named_captures, "harmony regexp named captures")     \
   V(harmony_regexp_property, "harmony unicode regexp property classes") \
+  V(harmony_function_tostring, "harmony Function.prototype.toString")   \
   V(harmony_class_fields, "harmony public fields in class literals")    \
   V(harmony_async_iteration, "harmony async iteration")                 \
   V(harmony_dynamic_import, "harmony dynamic import")                   \
@@ -214,7 +215,6 @@ DEFINE_IMPLICATION(es_staging, move_object_start)
 
 // Features that are complete (but still behind --harmony/es-staging flag).
 #define HARMONY_STAGED(V)                                                \
-  V(harmony_function_tostring, "harmony Function.prototype.toString")    \
   V(harmony_regexp_lookbehind, "harmony regexp lookbehind")              \
   V(harmony_restrictive_generators,                                      \
     "harmony restrictions on generator declarations")                    \

test/mjsunit/messages.js

@@ -371,7 +371,7 @@ test(function() {
 // kParenthesisInArgString
 test(function() {
   new Function(")", "");
-}, "Arg string terminates parameters early", SyntaxError);
+}, "Function arg string contains parenthesis", SyntaxError);
 
 // === ReferenceError ===