Change default of base_merge_drop_equivalent_class_axioms

[matentzn]

This is a temporary fix.

@dosumis wanted us to add "equivalent class removal" as a default to ODK, and in principle, we all agreed.

However, there is this is the fundamental issue:

1. we can only drop equivalent class axioms if we have a “true base”, i.e. the subclass axioms are asserted
2. we do not have true base yet - because its shipped the next ODK 1.5.

To not break everything everywhere with the new ODK release, we discussed to make this feature opt-in for now, and document it a bit bettter.