Remove built in "triggers"

Remove built in "triggers"

Originally, the idea was that clients would replicate "OS/tree"s from
a build server, but we'd run things like "ldconfig" on the client.
This was to allow adding e.g. the nVidia binary driver.

However, the triggers were the only thing in the system at the moment
that really had expected knowledge of the *contents* of the OS, like
the location of binaries.

For now, it's architecturally cleaner if we move the burden of
triggers to the tree builder (e.g. gnome-ostree or RPM).  Eventually
we may want OSTree to assist with this type of thing (perhaps
something like RPM %ghost), but this is the right thing to do now.