Only the pose library enables the asset shelf by default, but Blender
doesn't come with pose assets, they have to be created by the user. So
when entering pose mode with factory settings, the asset shelf will
currently be just a big empty bar. Plus, pose mode is used for posing
without pose assets quite a lot. So hide the asset shelf by default. The
pose library already shows a toggle to hide/unhide it where the previous
pose library UI was for discoverability.
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