get rid of lax's OpaqueParams class
get rid of lax's OpaqueParams class

That class was once used for smuggling consts into primitives for
lax.while_loop and lax.cond, but it's no longer needed because those
bind rules take constants as positional arguments now (and hence handle
closed-over tracers).

tensorflow/probability,google/jax,google/jax,google/jax,google/jax,tensorflow/probability