There is no other type alias with this naming convention. I believe it
is a remnant from the past, when we had e.g. MatrixN type aliases. But
these no longer exist, therefore I believe this alias should also be
removed, instead of updated to "MatrixViewMutN".
In the past, MatrixSliceMut did not have the same defaults. However,
for a while now MatrixSliceMut has served the exact same purpose
(in fact their definitions are identical), so we should
deprecate MatrixSliceMutMN.
