088b885087
Since 7b0ea0f1b4, brushes use the asset system and previews are stored
in the asset source blend files. The bundled ones are part of the
essentials asset library, see
`release/datafiles/assets/publis/brushes/essentials_brushes.blend`.
Note that this doesn't remove the toolbar icons for these brushes yet.
Initially reviewed in (but this commit contains further changes missing
in the PR):
https://projects.blender.org/blender/blender/pulls/123842