[Transforms] Remove unused forward declaration PostDominatorTree

The last use was removed by:

  commit b1f4168
  Author: Alexandros Lamprineas <alexandros.lamprineas@arm.com>
  Date:   Thu Jun 1 15:09:08 2023 +0100

llvm/include/llvm/Transforms/Utils/SCCPSolver.h

@@ -32,7 +32,6 @@ class Function;
 class GlobalVariable;
 class Instruction;
 class LLVMContext;
-class PostDominatorTree;
 class StructType;
 class TargetLibraryInfo;
 class Value;