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/TTY/TTY.cpp

@@ -286,7 +286,6 @@ void TTY::generate_signal(int signal)
         dbgln_if(TTY_DEBUG, "{}: Send signal {} to {}", tty_name(), signal, process);
         // FIXME: Should this error be propagated somehow?
         [[maybe_unused]] auto rc = process.send_signal(signal, nullptr);
-        return IterationDecision::Continue;
     });
 }