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. :^)

AK/FlyString.cpp

@@ -34,7 +34,7 @@
 
 namespace AK {
 
-struct FlyStringImplTraits : public AK::Traits<StringImpl*> {
+struct FlyStringImplTraits : public Traits<StringImpl*> {
     static unsigned hash(const StringImpl* s) { return s ? s->hash() : 0; }
     static bool equals(const StringImpl* a, const StringImpl* b)
     {