[SPARK-17653][SQL] Remove unnecessary distincts in multiple unions

## What changes were proposed in this pull request?

Currently for `Union [Distinct]`, a `Distinct` operator is necessary to be on the top of `Union`. Once there are adjacent `Union [Distinct]`,  there will be multiple `Distinct` in the query plan.

E.g.,

For a query like: select 1 a union select 2 b union select 3 c

Before this patch, its physical plan looks like:

    *HashAggregate(keys=[a#13], functions=[])
    +- Exchange hashpartitioning(a#13, 200)
       +- *HashAggregate(keys=[a#13], functions=[])
          +- Union
             :- *HashAggregate(keys=[a#13], functions=[])
             :  +- Exchange hashpartitioning(a#13, 200)
             :     +- *HashAggregate(keys=[a#13], functions=[])
             :        +- Union
             :           :- *Project [1 AS a#13]
             :           :  +- Scan OneRowRelation[]
             :           +- *Project [2 AS b#14]
             :              +- Scan OneRowRelation[]
             +- *Project [3 AS c#15]
                +- Scan OneRowRelation[]

Only the top distinct should be necessary.

After this patch, the physical plan looks like:

    *HashAggregate(keys=[a#221], functions=[], output=[a#221])
    +- Exchange hashpartitioning(a#221, 5)
       +- *HashAggregate(keys=[a#221], functions=[], output=[a#221])
          +- Union
             :- *Project [1 AS a#221]
             :  +- Scan OneRowRelation[]
             :- *Project [2 AS b#222]
             :  +- Scan OneRowRelation[]
             +- *Project [3 AS c#223]
                +- Scan OneRowRelation[]

## How was this patch tested?

Jenkins tests.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #15238 from viirya/remove-extra-distinct-union.