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/ResizeCorner.cpp

@@ -61,10 +61,6 @@ ResizeCorner::ResizeCorner()
     set_fixed_size(16, 18);
 }
 
-ResizeCorner::~ResizeCorner()
-{
-}
-
 void ResizeCorner::paint_event(PaintEvent& event)
 {
     Painter painter(*this);