Libraries: Use default constructors/destructors in LibGUI

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/Libraries/LibGUI/AutocompleteProvider.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, the SerenityOS developers.
+ * Copyright (c) 2020-2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -84,8 +84,6 @@ private:
     Vector<AutocompleteProvider::Entry> m_suggestions;
 };
 
-AutocompleteBox::~AutocompleteBox() { }
-
 AutocompleteBox::AutocompleteBox(TextEditor& editor)
     : m_editor(editor)
 {