Introduced rank should be rank or dissolved

[mdspan.submdspan.overview] introduces rank by "let"
in functions with a slices argument within [mdspan.submdspan].
rank is essentially sizeof...(slices)/sizeof...(SliceSpecifier).

In the same [mdspan.submdspan.overview],
it's then used to introduce another "let",
and never to be used again in [mdspan.submdspan].

There are actually many uses of sizeof...(SliceSpecifier) within [mdspan.submdspan]
that can be replaced with the introduced rank.
There's also one use of rank() (which is a different rank) while the introduced rank is in scope.

So I think either, the "let" rank should be renamed to rank or removed.
If not removed, we should consider replacing uses of sizeof...(SliceSpecifier) with rank.