LibSQL: Use compiler generated default functions

Problem:
- Clang ToT generates warnings due to user-declared functions causing
  the implicitly generated assignment operator to not be generated.

Solution:
- Declare the default constructor `= default`.
- Remove the default copy constructor declaration.

Userland/Libraries/LibSQL/Key.cpp

@@ -9,11 +9,6 @@
 
 namespace SQL {
 
-Key::Key()
-    : Tuple()
-{
-}
-
 Key::Key(TupleDescriptor const& descriptor)
     : Tuple(descriptor)
 {