Remove implementation of disabled opcodes

So we stop getting pull requests (like #2604) fixing problems with disabled Script opcodes.
A hard fork would be required to re-enable these, and if we ever did that we'd require extensive review and testing.

I double-checked to make sure the script_invalid.json unit tests make sure that the disabled opcodes are handled properly (script invalid even if the disabled opcodes are in the unexecuted branch of an OP_IF); they do.