Remove bogus code in oper_exact --- if it didn't find an exact

match then it tried for a self-commutative operator with the reversed input
data types.  This is pretty silly; there could never be such an operator,
except maybe in binary-compatible-type scenarios, and we have oper_inexact
for that.  Besides which, the oprsanity regress test would complain about
such an operator.  Remove nonfunctional code and simplify routine calling
convention accordingly.