[Serialization] Revert specialization for DenseMapInfo<GlobalDeclID>::getHashValue

The FIXME says to revert this when the underlying issue got fixed. And
now the underlying issue got fixed in
#95734. So I think it should be
fine to rever that one now.

Author: ChuanqiXu9

clang/include/clang/AST/DeclID.h

@@ -259,11 +259,7 @@ template <> struct DenseMapInfo<clang::GlobalDeclID> {
   }
 
   static unsigned getHashValue(const GlobalDeclID &Key) {
-    // Our default hash algorithm for 64 bits integer may not be very good.
-    // In GlobalDeclID's case, it is pretty common that the lower 32 bits can
-    // be same.
-    // FIXME: Remove this when we fix the underlying issue.
-    return llvm::hash_value(Key.getRawValue());
+    return DenseMapInfo<DeclID>::getHashValue(Key.getRawValue());
   }
 
   static bool isEqual(const GlobalDeclID &L, const GlobalDeclID &R) {