[mlir][linalg][nfc] Clean-up leftover code post #149156 (#150602)

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.