[IR] Remove unused forward declarations (NFC)

Author: kazutakahirata

llvm/lib/IR/Use.cpp

@@ -11,10 +11,6 @@
 
 namespace llvm {
 
-class User;
-template <typename> struct simplify_type;
-class Value;
-
 void Use::swap(Use &RHS) {
   if (Val == RHS.Val)
     return;