dead code cleanup

Signed-off-by: Jialiang Liang <jiallian@amazon.com>

Author: RyanL1997

core/src/main/java/org/opensearch/sql/calcite/plan/HighlightPushable.java

@@ -42,7 +42,6 @@
 import org.apache.logging.log4j.Logger;
 import org.opensearch.search.aggregations.AggregationBuilder;
 import org.opensearch.search.aggregations.bucket.composite.CompositeAggregationBuilder;
-import org.opensearch.sql.calcite.plan.HighlightPushable;
 import org.opensearch.sql.calcite.utils.OpenSearchTypeFactory;
 import org.opensearch.sql.calcite.utils.PPLHintUtils;
 import org.opensearch.sql.common.setting.Settings;
@@ -71,7 +70,7 @@
 
 /** The logical relational operator representing a scan of an OpenSearchIndex type. */
 @Getter
-public class CalciteLogicalIndexScan extends AbstractCalciteIndexScan implements HighlightPushable {
+public class CalciteLogicalIndexScan extends AbstractCalciteIndexScan {
   private static final Logger LOG = LogManager.getLogger(CalciteLogicalIndexScan.class);
 
   public CalciteLogicalIndexScan(
@@ -86,7 +85,6 @@ public CalciteLogicalIndexScan(
         new PushDownContext(osIndex));
   }
 
-  @Override
   public RelNode pushDownHighlight(List<String> highlightArgs) {
     RelDataTypeFactory.Builder schemaBuilder = getCluster().getTypeFactory().builder();
     schemaBuilder.addAll(getRowType().getFieldList());

opensearch/src/main/java/org/opensearch/sql/opensearch/storage/scan/CalciteLogicalIndexScan.java

@@ -572,7 +572,6 @@ highlightCommand
 highlightArg
    : STAR
    | stringLiteral
-   | fieldExpression
    ;
 
 expandCommand

ppl/src/main/antlr/OpenSearchPPLParser.g4

@@ -1294,10 +1294,8 @@ public UnresolvedPlan visitHighlightCommand(OpenSearchPPLParser.HighlightCommand
                 arg -> {
                   if (arg.STAR() != null) {
                     return "*";
-                  } else if (arg.stringLiteral() != null) {
-                    return StringUtils.unquoteText(arg.stringLiteral().getText());
                   } else {
-                    return arg.fieldExpression().getText();
+                    return StringUtils.unquoteText(arg.stringLiteral().getText());
                   }
                 })
             .collect(Collectors.toList());

ppl/src/main/java/org/opensearch/sql/ppl/parser/AstBuilder.java

@@ -1125,8 +1125,8 @@ public void testHighlightMultipleStringLiterals() {
   }
 
   @Test
-  public void testHighlightFieldExpression() {
-    assertEqual("source=t | highlight fieldname", highlight(relation("t"), List.of("fieldname")));
+  public void testHighlightFieldExpressionNotSupported() {
+    assertThrows(SyntaxCheckException.class, () -> plan("source=t | highlight fieldname"));
   }
 
   @Test