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. :^)

Kernel/SpinLock.h

@@ -69,7 +69,7 @@ public:
     }
 
 private:
-    AK::Atomic<BaseType> m_lock { 0 };
+    Atomic<BaseType> m_lock { 0 };
 };
 
 class RecursiveSpinLock {
@@ -121,7 +121,7 @@ public:
     }
 
 private:
-    AK::Atomic<FlatPtr> m_lock { 0 };
+    Atomic<FlatPtr> m_lock { 0 };
     u32 m_recursions { 0 };
 };