[es8] Remove syntactic tail calls support.

src/flag-definitions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file defines all of the flags.  It is separated into different section,
 // for Debug, Release, Logging and Profiling, etc.  To add a new flag, find the
 // correct section, and use one of the DEFINE_ macros, without a trailing ';'.
 //
 // This include does not have a guard, because it is a template-style include,
 // which can be included multiple times in different modes.  It expects to have
@@ skipping 180 matching lines @@
 // Activate on ClusterFuzz.
 DEFINE_IMPLICATION(es_staging, harmony_regexp_lookbehind)
 DEFINE_IMPLICATION(es_staging, move_object_start)
 
 // Features that are still work in progress (behind individual flags).
 #define HARMONY_INPROGRESS(V)                                           \
   V(harmony_array_prototype_values, "harmony Array.prototype.values")   \
   V(harmony_function_sent, "harmony function.sent")                     \
   V(harmony_sharedarraybuffer, "harmony sharedarraybuffer")             \
   V(harmony_simd, "harmony simd")                                       \
-  V(harmony_explicit_tailcalls, "harmony explicit tail calls")          \
   V(harmony_do_expressions, "harmony do-expressions")                   \
   V(harmony_restrictive_generators,                                     \
     "harmony restrictions on generator declarations")                   \
   V(harmony_regexp_named_captures, "harmony regexp named captures")     \
   V(harmony_regexp_property, "harmony unicode regexp property classes") \
   V(harmony_for_in, "harmony for-in syntax")                            \
   V(harmony_trailing_commas,                                            \
     "harmony trailing commas in function parameter lists")              \
   V(harmony_class_fields, "harmony public fields in class literals")
 
@@ skipping 996 matching lines @@
 #undef DEFINE_ALIAS_FLOAT
 #undef DEFINE_ALIAS_ARGS
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 
 #undef COMMA