core: drop specifier expansion when parsing transient dbus properties

core: drop specifier expansion when parsing transient dbus properties

Specifier expansion (much like C escape handling) should be a helper for
writing unit files, but should be nothing we do on programatic APIs. For
those, the client can do the necessary replacements anyway, and we
really should be careful with doing such string processing of data we
get via lower level programmatic APIs.

We currently do specifier expansion only for the env var transient unit
APIs, no other properties do this. Let's remove it here too, to be fully
systematic.

Yes, in a way this is API breakage, but then again this API isn't
documented yet, and an outlier, hence let's clear this up now, before it
is too late.