Originally, the SCRIPT_ENABLE_SCHNORR flag was added first without any
effect, then later diffs put in the functionality. This Diff removes the
effect of the SCRIPT_ENABLE_SCHNORR flag, but keeps the flag around.
Some tests have to be removed accordingly since they were specifically
testing flag-off behaviour that is no longer true.
(effectively this is a backing-out of D2455 and D2469)
Depends on D3253