Revert of [builtins] Migrate String.prototype.charCodeAt and String.prototype.charAt to TurboFan.

src/runtime/runtime-strings.cc

Index: src/runtime/runtime-strings.cc
diff --git a/src/runtime/runtime-strings.cc b/src/runtime/runtime-strings.cc
index 5b82ff5dcc42e049fcd42e242c2af55969524e7b..6786fa99fba0f92e1c41e5e228d9f6852349dd74 100644
--- a/src/runtime/runtime-strings.cc
+++ b/src/runtime/runtime-strings.cc
@@ -1273,13 +1273,6 @@
   return __RT_impl_Runtime_StringCharFromCode(Arguments(1, &code), isolate);
 }
 
-RUNTIME_FUNCTION(Runtime_ExternalStringGetChar) {
-  SealHandleScope shs(isolate);
-  DCHECK_EQ(2, args.length());
-  CONVERT_ARG_CHECKED(ExternalString, string, 0);
-  CONVERT_INT32_ARG_CHECKED(index, 1);
-  return Smi::FromInt(string->Get(index));
-}
 
 RUNTIME_FUNCTION(Runtime_OneByteSeqStringGetChar) {
   SealHandleScope shs(isolate);