Revert "core: Check unit start rate limiting earlier"

This reverts commit ed8fbbf1.

This was causing problems during boot, see
https://bodhi.fedoraproject.org/updates/FEDORA-2021-a1a52487e6,
https://bugzilla.redhat.com/show_bug.cgi?id=2013386.
https://github.com/systemd/systemd/issues/21025