[MINOR][MLLIB] deprecate setLabelCol in ChiSqSelectorModel

## What changes were proposed in this pull request?

Deprecate `labelCol`, which is not used by ChiSqSelectorModel.

Author: Xiangrui Meng <meng@databricks.com>

Closes #13823 from mengxr/deprecate-setLabelCol-in-ChiSqSelectorModel.

(cherry picked from commit d77c4e6)
Signed-off-by: Joseph K. Bradley <joseph@databricks.com>

Author: mengxr

mllib/src/main/scala/org/apache/spark/ml/feature/ChiSqSelector.scala

@@ -142,6 +142,7 @@ final class ChiSqSelectorModel private[ml] (
 
   /** @group setParam */
   @Since("1.6.0")
+  @deprecated("labelCol is not used by ChiSqSelectorModel.", "2.0.0")
   def setLabelCol(value: String): this.type = set(labelCol, value)
 
   @Since("2.0.0")