Remove the unused PDF20.hash method

This method was added in PR 4938, almost nine years ago, however it doesn't appear to ever have been used.
Given the similarities between the `PDF17` and `PDF20` classes, and how they're used, if the `PDF20.hash` method was actually necessary you'd also expect a similiar method in the `PDF17` class.

src/core/crypto.js

@@ -1321,10 +1321,6 @@ class PDF20 {
     return k.subarray(0, 32);
   }
 
-  hash(password, concatBytes, userBytes) {
-    return this._hash(password, concatBytes, userBytes);
-  }
-
   checkOwnerPassword(password, ownerValidationSalt, userBytes, ownerPassword) {
     const hashData = new Uint8Array(password.length + 56);
     hashData.set(password, 0);