Drop ForwardJournal= integration for coredumpctl and journalctl verbs

Drop ForwardJournal= integration for coredumpctl and journalctl verbs

Now that we have the sandbox verb, this is trivial to achieve with the
sandbox verb so let's keep the coredumpctl and journalctl verbs focused
solely on reading the journal and coredumps from the image itself and leave
inspecting the forwarded journal to mkosi sandbox.

Technically this breaks compat but I'm convinced nobody's actually using this
specific bit of integration so it shouldn't matter if we remove it.