[mlir][linalg] remove the -now- obsolete sparse support in linalg

All glue and clutter in the linalg ops has been replaced by proper
sparse tensor type encoding. This code is no longer needed. Thanks
to ntv@ for giving us a temporary home in linalg.

So long, and thanks for all the fish.

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D102098