simplify cotangent dropping in ad.backward_pass
simplify cotangent dropping in ad.backward_pass

Since variables in jaxprs are only assigned-to once, when we transpose
them we end up reading a variable's cotangent value only once. That
means we can pop the cotangent environment's reference to a cotangent
value in read_cotangent.

google/jax,google/jax,google/jax,google/jax