refactor: remove map_contains_key serde (apache#4703)

Remove the unreachable MapContainsKey serde and registry entry now that Spark rewrites map_contains_key to ArrayContains(MapKeys(...), key) before Comet serialization.

Co-authored-by: Codex <codex@openai.com>

Author: manuzhang

spark/src/main/scala/org/apache/comet/serde/QueryPlanSerde.scala

@@ -175,7 +175,6 @@ object QueryPlanSerde extends Logging with CometExprShim with CometTypeShim {
       classOf[MapEntries] -> CometMapEntries,
       classOf[MapValues] -> CometMapValues,
       classOf[MapFromArrays] -> CometMapFromArrays,
-      classOf[MapContainsKey] -> CometMapContainsKey,
       classOf[MapFromEntries] -> CometMapFromEntries,
       classOf[MapConcat] -> CometMapConcat,
       classOf[StringToMap] -> CometStrToMap,

spark/src/main/scala/org/apache/comet/serde/maps.scala

@@ -118,23 +118,6 @@ object CometMapFromArrays extends CometExpressionSerde[MapFromArrays] {
   }
 }
 
-object CometMapContainsKey extends CometExpressionSerde[MapContainsKey] {
-
-  override def convert(
-      expr: MapContainsKey,
-      inputs: Seq[Attribute],
-      binding: Boolean): Option[ExprOuterClass.Expr] = {
-    // Replace with array_has(map_keys(map), key)
-    val mapExpr = exprToProtoInternal(expr.left, inputs, binding)
-    val keyExpr = exprToProtoInternal(expr.right, inputs, binding)
-
-    val mapKeysExpr = scalarFunctionExprToProto("map_keys", mapExpr)
-
-    val mapContainsKeyExpr = scalarFunctionExprToProto("array_has", mapKeysExpr, keyExpr)
-    optExprWithFallbackReason(mapContainsKeyExpr, expr, expr.children: _*)
-  }
-}
-
 object CometMapFromEntries
     extends CometScalarFunction[MapFromEntries]("map_from_entries")
     with CodegenDispatchFallback {