Remove legacy type checking code and update documentation

Delete PPLTypeChecker, CalciteFuncSignature, CoercionUtils (moved to
validate/ package), and OpenSearchSparkSqlDialect (moved to core/)
which are no longer needed in their original locations after migrating
to Calcite's validation system. Update LambdaUtils and CalcitePPLNoMvTest
to match final PR state. Update user-facing documentation for the new
behavior.

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

core/src/main/java/org/opensearch/sql/expression/function/CalciteFuncSignature.java

@@ -6,17 +6,6 @@
 package org.opensearch.sql.expression.function.CollectionUDF;
 
 import java.math.BigDecimal;
-import java.util.ArrayList;
-import java.util.List;
-import java.util.Map;
-import org.apache.calcite.rel.type.RelDataType;
-import org.apache.calcite.rel.type.RelDataTypeFactory;
-import org.apache.calcite.rex.RexCall;
-import org.apache.calcite.rex.RexCallBinding;
-import org.apache.calcite.rex.RexLambda;
-import org.apache.calcite.rex.RexLambdaRef;
-import org.apache.calcite.rex.RexNode;
-import org.apache.calcite.sql.type.SqlReturnTypeInference;
 import org.apache.calcite.sql.type.SqlTypeName;
 
 public class LambdaUtils {
@@ -37,58 +26,4 @@ public static Object transferLambdaOutputToTargetType(Object candidate, SqlTypeN
       return candidate;
     }
   }
-
-  public static RelDataType inferReturnTypeFromLambda(
-      RexLambda rexLambda, Map<String, RelDataType> filledTypes, RelDataTypeFactory typeFactory) {
-    RexCall rexCall = (RexCall) rexLambda.getExpression();
-    SqlReturnTypeInference returnInfer = rexCall.getOperator().getReturnTypeInference();
-    List<RexNode> lambdaOperands = rexCall.getOperands();
-    List<RexNode> filledOperands = new ArrayList<>();
-    for (RexNode rexNode : lambdaOperands) {
-      if (rexNode instanceof RexLambdaRef rexLambdaRef) {
-        if (rexLambdaRef.getType().getSqlTypeName() == SqlTypeName.ANY) {
-          filledOperands.add(
-              new RexLambdaRef(
-                  rexLambdaRef.getIndex(),
-                  rexLambdaRef.getName(),
-                  filledTypes.get(rexLambdaRef.getName())));
-        } else {
-          filledOperands.add(rexNode);
-        }
-      } else if (rexNode instanceof RexCall) {
-        filledOperands.add(
-            reInferReturnTypeForRexCallInsideLambda((RexCall) rexNode, filledTypes, typeFactory));
-      } else {
-        filledOperands.add(rexNode);
-      }
-    }
-    return returnInfer.inferReturnType(
-        new RexCallBinding(typeFactory, rexCall.getOperator(), filledOperands, List.of()));
-  }
-
-  public static RexCall reInferReturnTypeForRexCallInsideLambda(
-      RexCall rexCall, Map<String, RelDataType> argTypes, RelDataTypeFactory typeFactory) {
-    List<RexNode> filledOperands = new ArrayList<>();
-    List<RexNode> rexCallOperands = rexCall.getOperands();
-    for (RexNode rexNode : rexCallOperands) {
-      if (rexNode instanceof RexLambdaRef rexLambdaRef) {
-        filledOperands.add(
-            new RexLambdaRef(
-                rexLambdaRef.getIndex(),
-                rexLambdaRef.getName(),
-                argTypes.get(rexLambdaRef.getName())));
-      } else if (rexNode instanceof RexCall) {
-        filledOperands.add(
-            reInferReturnTypeForRexCallInsideLambda((RexCall) rexNode, argTypes, typeFactory));
-      } else {
-        filledOperands.add(rexNode);
-      }
-    }
-    RelDataType returnType =
-        rexCall
-            .getOperator()
-            .inferReturnType(
-                new RexCallBinding(typeFactory, rexCall.getOperator(), filledOperands, List.of()));
-    return rexCall.clone(returnType, filledOperands);
-  }
 }