[turbofan] Remove outdated forward declarations.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/869723003

Cr-Commit-Position: refs/heads/master@{#26240}

src/compiler/schedule.h

@@ -16,8 +16,6 @@ namespace compiler {
 // Forward declarations.
 class BasicBlock;
 class BasicBlockInstrumentor;
-class CodeGenerator;  // Because of a namespace bug in clang.
-class ConstructScheduleData;
 class Node;