After #9737 and #9828 there is still some unused code that we might want to remove.

#9828 (comment)

We implement the Description trait for Sapling types; however, that is not required anymore since we switched to the sapling_crypto verifier (#9737). The primary_inputs() methods is no longer used, and the proof() method can be bypassed by simply accessing the proof field when needed.

This allows us removing a bunch of code which is no longer needed, decreasing maintenance burden.