[SPARK-5755] [SQL] remove unnecessary Add

[adrian-wang]

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

[SparkQA]

Test build #27322 has started for PR 4551 at commit 0821ae4.

• This patch merges cleanly.

[SparkQA]

Test build #27322 has finished for PR 4551 at commit 0821ae4.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • trait DataFrame extends RDDApi[Row] with Serializable

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/27322/
Test PASSed.