Remove plainly_exist_1 from the BI axioms.

See merge request FP/iris-coq!95

To be consistent with the lemma for the persistence modality.

Note sure whether these premises are the weakest possible.

Also, persistent stuff goes before plain stuff.

We do not have a notation for `bi_affinely` either, so this is at
least consistent.

Use `simpl` in `wp_` tactics just in the expression

Closes #113

See merge request FP/iris-coq!94