Get rid of the unused native code StringAddStub.

src/flag-definitions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 331 matching lines @@
 DEFINE_bool(block_concurrent_recompilation, false,
             "block queued jobs until released")
 DEFINE_bool(concurrent_osr, false,
             "concurrent on-stack replacement")
 DEFINE_implication(concurrent_osr, concurrent_recompilation)
 
 DEFINE_bool(omit_map_checks_for_leaf_maps, true,
             "do not emit check maps for constant values that have a leaf map, "
             "deoptimize the optimized code if the layout of the maps changes.")
 
-DEFINE_bool(new_string_add, true, "enable new string addition")
-
 // Profiler flags.
 DEFINE_int(frame_count, 1, "number of stack frames inspected by the profiler")
            // 0x1700 fits in the immediate field of an ARM instruction.
 DEFINE_int(interrupt_budget, 0x1700,
            "execution budget before interrupt is triggered")
 DEFINE_int(type_info_threshold, 25,
            "percentage of ICs that must have type info to allow optimization")
 DEFINE_int(self_opt_count, 130, "call count before self-optimization")
 
 DEFINE_bool(trace_opt_verbose, false, "extra verbose compilation tracing")
@@ skipping 528 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