[mlir][Linalg] Drop output tensor from `linalg.pad_tensor` op.

The output tensor was added for tiling purposes. With use of
`TilingInterface` for tiling pad operations, there is no need for an
explicit operand for the shape of result of `linalg.pad_tensor`
op. The interface allows the tiling pattern to query the value that
can be used for the "init" needed for tiling dynamically.

Differential Revision: https://reviews.llvm.org/D108613