The concept is flawed, and mostly useless. Let's finally remove it.
It has been deprecated since 90a2ec10f2 (6
years ago) and we started to warn since
55dadc5c57 (1.5 years ago).
Let's get rid of it altogether.
Prompted by the discussions in #15180.
This is a bit more complex than I hoped, since for PID 1 we need to pass
in the synethetic environment block in we generate on demand.
It's easy to add, and should be pretty useful, in particular as in
AssertPathIsEncrypted= as it can be used for checking that
some path is encrypted before some service is invoked that might want to
place secure material there.
