AK+LibC: Remove SERENITY_LIBC_BUILD guard around <initializer_list>

This was required before commit 5f724b6ca1
when we were building LibC before libstdc++ headers were available in
the sysroot. However as noted in that commit, we never actually needed
to be building LibC before libstdc++, so we can go ahead and remove this
ancient hack.

AK/Vector.h

@@ -17,13 +17,7 @@
 #include <AK/Traits.h>
 #include <AK/TypedTransfer.h>
 #include <AK/kmalloc.h>
-
-// NOTE: We can't include <initializer_list> during the toolchain bootstrap,
-//       since it's part of libstdc++, and libstdc++ depends on LibC.
-//       For this reason, we don't support Vector(initializer_list) in LibC.
-#ifndef SERENITY_LIBC_BUILD
-#    include <initializer_list>
-#endif
+#include <initializer_list>
 
 #ifndef __serenity__
 #    include <new>
@@ -65,14 +59,12 @@ public:
     {
     }
 
-#ifndef SERENITY_LIBC_BUILD
     Vector(std::initializer_list<T> list) requires(!IsLvalueReference<T>)
     {
         ensure_capacity(list.size());
         for (auto& item : list)
             unchecked_append(item);
     }
-#endif
 
     Vector(Vector&& other)
         : m_size(other.m_size)