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

Previously, iFrame 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 (closed)