So I discover the concept of obsolete data in GIMP where some data can
be apparently unavailable in the GUI, but still selectable by script.
This somehow reverts commit 26cb87aa
then moves the brushes into the obsolete section instead of simple
deletion. This way, any script using the said brushes won't break at
least.

We will definitely remove the brushes for 3.0, but for now it's better
to keep scripts as compatible as possible.