test: Remove unused, undocumented and misleading CScript.add

Summary:
CScript is a subclass of bytes. This removes the overloaded implementation of CScript.__add__ and ensures that it raises an error instead of calling bytes.__add__.

This is a backport of Core PR18732

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8960