[mlir] cleanup transform payload setting

Before the multi-handle relaxation, the transform interpreter did not
actually set payload for a handle that had no further uses as a hacky
mechanism to work around the multi-handle problem. This is no longer
necessary and can be removed to avoid debugging surprises when a handle
has no payload even when its producing op assigned it.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D135585