remove most 3.10 bw compat Related to #3799117. The boxes and entityvcomponent objects cannot really be removed as they are still used throughout the code base (and possible cubes). This may be caused by a non-working deprecation and needs some more work.

What's new in CubicWeb 3.20
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Deprecated Code Drops
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* most 3.10 backward compat is gone (box related objects are still
  there, but you are encouraged to not depend on them)