Stop turning off bit 1 in rv2cv private flags

Stop turning off bit 1 in rv2cv private flags

newUNOP turns on that bit by default, to indicate that the op
has one kid.

rv2cv was using that bit for something else, so three code paths had
to turn it off.

Two commits ago, rv2cv stopped using that bit, so turning it off
is no longer necessary.  It was because the flag using that bit,
OPpMAY_RETURN_CONSTANT, needed to be passed to newUNOP, that it had
to change.