Face maps were added as a prototype of a new rigging solution during
2.8 development. Their storage is redundant with the newer generic
attribute system (specifically with integer face attributes), and
they were never used much. This commit removes the face map list
and converts the storage to an attribute with the name `face_maps`.
There is nowhere to store the face map names anymore, so those
are not kept.
It probably still makes sense to have a feature like mesh face gizmo
selection for rigging. But the design and implementation woulds likely
have to change significantly, including possibly changing the storage
type, and making use of the generic attribute system instead of a
special type.
See #105317 for more discussion.
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