Libraries: Use default constructors/destructors in LibGfx

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/LibGfx/WindowTheme.cpp

@@ -15,8 +15,4 @@ WindowTheme& WindowTheme::current()
     return theme;
 }
 
-WindowTheme::~WindowTheme()
-{
-}
-
 }