Because CHECKDATASIG uses a previouly invalid opcode number, it does
not appear in any executed branches of scripts prior its introduction in the
Nov 15, 2018 Upgrade. This means that the interpreter can treat it as if
it has always been valid.
Before this change, the SCRIPT_ENABLE_CHECKDATASIG flag had both a permissive
(enabling the opcode) and a restrictive (enforcing siops counting) meaning.
This change simplifies the meaning of the flag to be restrictive only.