[mlir][sparse][nfc] Fix memory leak in sparse sampled matmul integration test

https://reviews.llvm.org/D130023 added a memory leak in sparse_sampled_matmul.mlir
This diff fixes the memory leak.

Testing: Ran integration tests after building with -DLLVM_USE_SANITIZER=Address flag.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D130428