fs: get rid of package-provided post-fs hooks

fs: get rid of package-provided post-fs hooks

Now that the pre-fs ones are run on a transient copy of target/, the
post-fs hooks are no longer needed because we no longer need to restore
the target/ directory as it is only a internal copy.

Remove support for the post-fs hooks, and update the sole package using
them.

We do not add a legacy check because this was mostly a purely-internal
detail that was never really exposed nor documented.

Signed-off-by: "Yann E. MORIN" <[email protected]>
Cc: Thomas Petazzoni <[email protected]>
Cc: Arnout Vandecappelle <[email protected]>
Signed-off-by: Peter Korsgaard <[email protected]>