inboxwritable: drop {-importer} cyclic reference

InboxWritable caching the result of ->importer leads to a
circular references with returned (V2Writable|Import) object
holds onto the calling InboxWritable object.

With public-inbox-watch, this leads to a memory leak if a user
is reloading via SIGHUP after a message is imported (it would
only become noticeable with SIGHUPs after every message imported).

I would not expect anybody to to notice this in real-world
usage.  I only noticed this since I was making -xcpdb suitable
for long-lived process use (e.g. "mod_perl style") and a flock
remained unreleased on v1 inboxes after resharding.

WatchMaildir (used by -watch) already handles caching of the
importer object itself, and all of our other real-world uses of
->importer are short-lived or designed for batch scripts, so
there's no need to cache the importer result internally.

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