Remove now-unused string_iterator symbols

These were used by the JS implementation, but are now dead code.

R=caitp@igalia.com
BUG=v8:5388

Review-Url: https://codereview.chromium.org/2410533002
Cr-Commit-Position: refs/heads/master@{#40140}

src/heap-symbols.h

@@ -217,8 +217,6 @@
   V(sealed_symbol)                          \
   V(stack_trace_symbol)                     \
   V(strict_function_transition_symbol)      \
-  V(string_iterator_iterated_string_symbol) \
-  V(string_iterator_next_index_symbol)      \
   V(uninitialized_symbol)
 
 #define PUBLIC_SYMBOL_LIST(V)                \