src/plugin.c: Don't require write callbacks to be present.

src/plugin.c: Don't require write callbacks to be present.

plugin_dispatch_values() used to fail, if no write callbacks had been
registered. As that function is used to update the cache and check
threshold as well, this limited the flexibility of collectd setups. Thus,
the error message has been downgraded to a one-time complaint.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>