Revert "[SPARK-5244] [SQL] add coalesce() in sql parser"

This reverts commit 812d3679.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/SqlParser.scala

@@ -50,7 +50,6 @@ class SqlParser extends AbstractSparkSQLParser {
   protected val CACHE = Keyword("CACHE")
   protected val CASE = Keyword("CASE")
   protected val CAST = Keyword("CAST")
-  protected val COALESCE = Keyword("COALESCE")
   protected val COUNT = Keyword("COUNT")
   protected val DECIMAL = Keyword("DECIMAL")
   protected val DESC = Keyword("DESC")
@@ -296,7 +295,6 @@ class SqlParser extends AbstractSparkSQLParser {
       { case s ~ p => Substring(s, p, Literal(Integer.MAX_VALUE)) }
     | (SUBSTR | SUBSTRING) ~ "(" ~> expression ~ ("," ~> expression) ~ ("," ~> expression) <~ ")" ^^
       { case s ~ p ~ l => Substring(s, p, l) }
-    | COALESCE ~ "(" ~> repsep(expression, ",") <~ ")" ^^ { case exprs => Coalesce(exprs) }
     | SQRT  ~ "(" ~> expression <~ ")" ^^ { case exp => Sqrt(exp) }
     | ABS   ~ "(" ~> expression <~ ")" ^^ { case exp => Abs(exp) }
     | ident ~ ("(" ~> repsep(expression, ",")) <~ ")" ^^

sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala

@@ -86,15 +86,6 @@ class SQLQuerySuite extends QueryTest with BeforeAndAfterAll {
     setConf(SQLConf.CODEGEN_ENABLED, originalValue.toString)
   }
 
-  test("Add Parser of SQL COALESCE()") {
-    checkAnswer(
-      sql("""SELECT COALESCE(1, 2)"""),
-      1)
-    checkAnswer(
-      sql("SELECT COALESCE(null, null, null)"),
-      null)
-  }
-
   test("SPARK-3176 Added Parser of SQL LAST()") {
     checkAnswer(
       sql("SELECT LAST(n) FROM lowerCaseData"),