Revert "systemctl: when removing enablement or mask symlinks, cover both /run and /etc"

Having systemctl disable/unmask remove all symlinks in /etc and /run is
unintuitive and breaks existing use cases.
systemctl should behave symmetrically.
A "systemctl --runtime unmask" should undo a "systemctl --runtime mask"
action.
Say you have a service, which was masked by the admin in /etc.
If you temporarily want to mask the execution of the service (say in a
script), you'd create a runtime mask via "systemctl --runtime mask".
It is is now no longer possible to undo this temporary mask without
nuking the admin changes, unless you start rm'ing files manually.

While it is useful to be able to remove all enablement/mask symlinks in
one go, this should be done via a separate command line switch, like
"systemctl --all unmask".

This reverts commit 4910b350.

Fixes: #9393