[mlir][sparse] remove singleton dimension level type (for now)

Although we have plans to support this, and many other, dimension level type(s), currently the tag is not supported. It will be easy to add this back once support is added.

NOTE: based on discussion in https://discourse.llvm.org/t/overcoming-sparsification-limitation-on-level-types/62585

https://github.com/llvm/llvm-project/issues/51658

Reviewed By: Peiming

Differential Revision: https://reviews.llvm.org/D131002