Remove notations for bi_bare and bi_persistently.
(□ P) now means (bi_bare (bi_persistently P)). This is motivated by the fact that these two modalities are rarely used separately. In the case of an affine BI, we keep the □ notation. This means that a bi_bare is inserted each time we use □. Hence, a few adaptations need to be done in the proof mode class instances.
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