[uicfg] remove pre 3.6 bw compat structure (prepares #2406609)
This should have been done in a673d1d9a738.
What's new in CubicWeb 3.16?
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New functionnalities
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* Add a new dataimport store (`SQLGenObjectStore`). This store enables a fast
import of data (entity creation, link creation) in CubicWeb, by directly
flushing information in SQL. This may only be used with PostgreSQL, as it
requires the 'COPY FROM' command.
API changes
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Unintrusive API changes
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* Drop of `cubicweb.web.uicfg.AutoformSectionRelationTags.bw_tag_map`,
deprecated since 3,6.
User interface changes
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* Remove changelog view, as nor cubicweb nor known cubes/applications were properly
feeding related files