[Utils] Remove unused forward declaration CallGraph

The last use was removed by:

  commit fa6ea7a
  Author: Arthur Eubanks <aeubanks@google.com>
  Date:   Mon Mar 20 11:18:35 2023 -0700

llvm/include/llvm/Transforms/Utils/Cloning.h

@@ -33,7 +33,6 @@ class AAResults;
 class AllocaInst;
 class BasicBlock;
 class BlockFrequencyInfo;
-class CallGraph;
 class DebugInfoFinder;
 class DominatorTree;
 class Function;