[SPARK-5755] [SQL] remove unnecessary Add

    explain extended select +key from src;
before:
== Parsed Logical Plan ==
'Project [(0 + 'key) AS _c0#8]
 'UnresolvedRelation [src], None

== Analyzed Logical Plan ==
Project [(0 + key#10) AS _c0#8]
 MetastoreRelation test, src, None

== Optimized Logical Plan ==
Project [(0 + key#10) AS _c0#8]
 MetastoreRelation test, src, None

== Physical Plan ==
Project [(0 + key#10) AS _c0#8]
 HiveTableScan [key#10], (MetastoreRelation test, src, None), None

after this patch:
== Parsed Logical Plan ==
'Project ['key]
 'UnresolvedRelation [src], None

== Analyzed Logical Plan ==
Project [key#10]
 MetastoreRelation test, src, None

== Optimized Logical Plan ==
Project [key#10]
 MetastoreRelation test, src, None

== Physical Plan ==
HiveTableScan [key#10], (MetastoreRelation test, src, None), None

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #4551 from adrian-wang/positive and squashes the following commits:

0821ae4 [Daoyuan Wang] remove unnecessary Add