Remove MathPowHalf builtin id

Because it's dead.

Change-Id: I0c5578f57b1fd5ba5960e6070fb2828b6f0822be
Reviewed-on: https://chromium-review.googlesource.com/1084837Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53489}

src/objects.h

@@ -3204,9 +3204,6 @@ enum BuiltinFunctionId {
   FUNCTIONS_WITH_ID_LIST(DECL_FUNCTION_ID)
       ATOMIC_FUNCTIONS_WITH_ID_LIST(DECL_FUNCTION_ID)
 #undef DECL_FUNCTION_ID
-  // Fake id for a special case of Math.pow. Note, it continues the
-  // list of math functions.
-  kMathPowHalf,
   // These are manually assigned to special getters during bootstrapping.
   kArrayBufferByteLength,
   kArrayBufferIsView,