[mlir][sparse] remove the MLIR PyTACO tests (#66302)

Rationale:

This test was really fun to compare the MLIR sparsifier with TACO using
the PyTACO format. However, the underlying mechanism is rapidly growing
outdated with our recent developments. Rather than maintaining the old
code, we are moving toward the newer, better approaches. So if you are
sad this is gone, stay tuned, something better is coming!