Make `iFrame` "less" smart w.r.t. clean up of modalities.

Previously, if would "cleanup" `<affine>` and `□` if the result after framing
is affine and intuitionistic, respectively. This behavior was inconsistent,
since similar "cleanup" was not performed for `<absorbing>` and `<persistent>`.
This MR thus removes this "cleanup" of modalities. It now consistently removes
the modalities `<affine>`, `<absorbing>, `<persistent>` and `□` only if the
result after framing is `True` or `emp`.

Since `iFrame` is already very complicated, and since its performance is
sometimes suboptimal in bigger developments, @jung and I believed doing
fewer "smart" things is better than the alternative, namely performing doing
sophisticated "cleanup" for all modalities, which is presented in
!450