[mlir][linalg] Clarify comments and remove outdated TODO (nfc) (#171695)
The TODO being removed was originally added at my request in:
* https://github.com/llvm/llvm-project/pull/160246
Upon revisiting the issue, it becomes clear that implementing the TODO
would require supporting IR of the following form (note the static tile
size `8` and the corresponding dynamic dimension in the result type):
```mlir
%pack = linalg.pack %src
padding_value(%c5 : i32)
inner_dims_pos = [0, 1]
inner_tiles = [8, 1]
into %dest : tensor<?x?xi32> -> tensor<1x1x?x1xi32>
```
Allowing this would be invalid, as discussed here:
* https://discourse.llvm.org/t/tensor-ops-with-dynamic-sizes-which-behaviour-is-more-correct