ggml : fix assert in ggml_set_unary_op

[slaren]

The assert in ggml_set_unary_op was an assignment instead of a comparison. Changing it to a comparison wouldn't be enough either, because at the time this function is called, the op still isn't set.

There could be several ways to fix this, I chose to remove ggml_set_unary_op entirely because I don't think we will ever need to use this function outside ggml_unary_impl, and it breaks the pattern of the way that op_params is set in other ops.