Everywhere: Remove a bunch of redundant 'AK::' namespace prefixes

This is basically just for consistency, it's quite strange to see
multiple AK container types next to each other, some with and some
without the namespace prefix - we're 'using AK::Foo;' a lot and should
leverage that. :^)

Userland/Libraries/LibGUI/Variant.cpp

@@ -257,7 +257,7 @@ Variant& Variant::operator=(Variant&& other)
     if (&other == this)
         return *this;
     clear();
-    move_from(AK::move(other));
+    move_from(move(other));
     return *this;
 }