Build: spec: prevent -remote pkg dragging systemd into system without it

At least since 1.1.17, pacemaker_remoted can stand for full-fledged PID1
hence it doubly does not make sense for its installation, e.g. in the
container, to bring systemd in (it may be blocked by other means, but
still). No need to modify anything about "%{post,...} remote" as
"%systemd_{post,...}" are resolved to silent, non-fatal systemctl
invocations.

Also simplify apply-when-defined for the main package in a similar way
already applied.

References:
https://github.com/systemd/systemd/commit/2424b6bd7
https://github.com/systemd/systemd/pull/3776