DevTools: Use default constructors/destructors

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#cother-other-default-operation-rules

"The compiler is more likely to get the default semantics right and
you cannot implement these functions better than the compiler."

Userland/DevTools/Profiler/SignpostsModel.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -15,10 +16,6 @@ SignpostsModel::SignpostsModel(Profile& profile)
 {
 }
 
-SignpostsModel::~SignpostsModel()
-{
-}
-
 int SignpostsModel::row_count(GUI::ModelIndex const&) const
 {
     return m_profile.filtered_signpost_indices().size();