Quieter deprecation warnings, at least for `beet completion`

https://bbs.archlinux.org/viewtopic.php?id=195893

The warning should only be displayed when beets itself is started. As of now with every start of bash the warning message is shown without any hint where it originates from.

Start by reproducing the warning during a new bash session and compare it with running beets itself, focusing on the `beet completion` entry point. Trace where the deprecation warning originates and verify that it appears only when beets is started, not on every shell start.

Written by the indexing model from the issue text.