Remove unused construct_tag_hash and Ciphertext.check()

[cygnusv]

There's currently 2 versions of construct_tag_hash(), and on of them seems incorrect:

ferveo/tpke/src/ciphertext.rs

Lines 45 to 51 in 434fd5d

	fn construct_tag_hash(&self) -> Result<E::G2Affine> {
	let mut hash_input = Vec::<u8>::new();
	self.commitment.serialize_compressed(&mut hash_input)?;
	hash_input.extend_from_slice(&self.ciphertext);
	hash_to_g2(&hash_input)
	}

As you can see, the AAD is not there. It's also just used here:

ferveo/tpke/src/ciphertext.rs

Line 36 in 434fd5d

let hash_g2 = E::G2Prepared::from(self.construct_tag_hash()?);

This is part the function Ciphertext.check(), which is unused.

I propose removing both.