[mlir][linalg][nfc] Clean-up leftover code post #149156 (#151334)
In https://github.com/llvm/llvm-project/pull/149156, I ensured that we
no longer generate spurious `tensor.empty` ops when vectorizing
`linalg.unpack`.
This follow-up removes leftover code that is now redundant but was
missed in the original PR and in #150602 that was also meant to clean-up
left-over code.
Note, this is removing code to compute "write-vector-sizes". Instead,
these are fully inferred from previous Ops.