AK+Kernel+LibELF: Remove the need for IteratorDecision::Continue

By constraining two implementations, the compiler will select the best
fitting one. All this will require is duplicating the implementation and
simplifying for the `void` case.

This constraining also informs both the caller and compiler by passing
the callback parameter types as part of the constraint
(e.g.: `IterationFunction<int>`).

Some `for_each` functions in LibELF only take functions which return
`void`. This is a minimal correctness check, as it removes one way for a
function to incompletely do something.

There seems to be a possible idiom where inside a lambda, a `return;` is
the same as `continue;` in a for-loop.

Kernel/Scheduler.cpp

@@ -584,7 +584,7 @@ void dump_thread_list()
         return thread.get_register_dump_from_stack().eip;
     };
 
-    Thread::for_each([&](Thread& thread) -> IterationDecision {
+    Thread::for_each([&](Thread& thread) {
         switch (thread.state()) {
         case Thread::Dying:
             dbgln("  {:14} {:30} @ {:04x}:{:08x} Finalizable: {}, (nsched: {})",
@@ -605,8 +605,6 @@ void dump_thread_list()
                 thread.times_scheduled());
             break;
         }
-
-        return IterationDecision::Continue;
     });
 }