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/HashMap.h

@@ -9,13 +9,7 @@
 #include <AK/HashTable.h>
 #include <AK/Optional.h>
 #include <AK/Vector.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 HashMap(initializer_list) in LibC.
-#ifndef SERENITY_LIBC_BUILD
-#    include <initializer_list>
-#endif
+#include <initializer_list>
 
 namespace AK {
 
@@ -38,14 +32,12 @@ public:
 
     HashMap() = default;
 
-#ifndef SERENITY_LIBC_BUILD
     HashMap(std::initializer_list<Entry> list)
     {
         ensure_capacity(list.size());
         for (auto& item : list)
             set(item.key, item.value);
     }
-#endif
 
     [[nodiscard]] bool is_empty() const
     {