Remove unnecessary includes of v8 headers from platform files

src/codegen.h

Index: src/codegen.h
diff --git a/src/codegen.h b/src/codegen.h
index 6b5f9513eaf4f6ac19bbc26e10174fe069c884b7..936b955863ed36c4622e6a86ac9cf03093c19d48 100644
--- a/src/codegen.h
+++ b/src/codegen.h
@@ -113,15 +113,6 @@ class CodeGenerator {
 };
 
 
-// Results of the library implementation of transcendental functions may differ
-// from the one we use in our generated code.  Therefore we use the same
-// generated code both in runtime and compiled code.
-typedef double (*UnaryMathFunction)(double x);
-
-UnaryMathFunction CreateExpFunction();
-UnaryMathFunction CreateSqrtFunction();
-
-
 class ElementsTransitionGenerator : public AllStatic {
  public:
   // If |mode| is set to DONT_TRACK_ALLOCATION_SITE,