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)