[mlir][sparse][gpu] cleanup GPUDataTransferStrategy (#71615)

The flag seems to be doing practically the same thing for zero cost and
pinned dma. In addition, the register host is not truly the right zero
cost mechanism according to Thomas. So we are simplifying the setup for
now, until we have a better definition for what to implement and test.
    
https://github.com/llvm/llvm-project/issues/64316