cubicweb #3749736 [hooks] enabled_category should NOT be a predicate [open]

enabled/disabled should be computed in the prune phase, once per transaction (?) and definitely not a second time as a predicate in _select_best.

done in<not specified>
load left0.200
closed by<not specified>
patch[hooks] remove the enabled_cat predicate [rejected][hooks] stop using the Hook.__select__ base predicate, which is deprecated. [rejected][hooks] replace a comment with an accurate note [rejected]