unify the terms NDV and cardinality in the optimizer

[qw4990]

Enhancement

Unify the terms NDV and cardinality in the optimizer.
In the optimizer, both terms NDV and cardinality are used to refer to the same meaning the number of distinct values.
In many other DBs, cardinality also means the estimated number of rows returned by a step or an operator.
So to make it clear and unified, we decided to remove cardinality and only use NDV to refer to this meaning in the optimizer.