[SPARK-14205][SQL] remove trait Queryable

[cloud-fan]

What changes were proposed in this pull request?

After DataFrame and Dataset are merged, the trait Queryable becomes unnecessary as it has only one implementation. We should remove it.

How was this patch tested?

existing tests.

[cloud-fan]

cc @liancheng @rxin @gatorsmile

[SparkQA]

Test build #54325 has finished for PR 12001 at commit 68257a5.

• This patch fails MiMa tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

[error]  * the type hierarchy of class org.apache.spark.sql.Dataset is different in current version. Missing types {org.apache.spark.sql.execution.Queryable}
[error]    filter with: ProblemFilters.exclude[MissingTypesProblem]("org.apache.spark.sql.Dataset")

[rxin]

LGTM (except the mima issue).

[liancheng]

LGTM

[gatorsmile]

LGTM

[SparkQA]

Test build #54331 has finished for PR 12001 at commit 9bd20aa.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

[error] /home/jenkins/workspace/SparkPullRequestBuilder/sql/core/src/test/scala/org/apache/spark/sql/QueryTest.scala:31: object Queryable is not a member of package org.apache.spark.sql.execution
[error] import org.apache.spark.sql.execution.{LogicalRDD, Queryable}
[error]        ^
[error] /home/jenkins/workspace/SparkPullRequestBuilder/sql/core/src/test/scala/org/apache/spark/sql/QueryTest.scala:185: not found: type Queryable
[error]   def assertCached(query: Queryable, numCachedTables: Int = 1): Unit = {
[error]                           ^
[error] /home/jenkins/workspace/SparkPullRequestBuilder/sql/core/src/test/scala/org/apache/spark/sql/QueryTest.scala:291: not found: type Queryable
[error]   def assertEmptyMissingInput(query: Queryable): Unit = {
[error]                                      ^

[SparkQA]

Test build #54382 has finished for PR 12001 at commit 9d0497f.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

Thanks - merging in master.