sql/schemachanger: drop index notices shouldn't always be generated

Previously, the declarative schema changer notices
were always generated even if no indexes were dropped.
This was inadequate because some of these should have
been reported if any work was done. To address this, this
patch will detect if any indexes were actually dropped.

Release note: None

pkg/sql/pgwire/testdata/pgtest/notice

@@ -55,7 +55,7 @@ Query {"String": "DROP INDEX t_x_idx"}
 until crdb_only
 CommandComplete
 ----
-{"Severity":"NOTICE","SeverityUnlocalized":"NOTICE","Code":"00000","Message":"the data for dropped indexes is reclaimed asynchronously","Detail":"","Hint":"The reclamation delay can be customized in the zone configuration for the table.","Position":0,"InternalPosition":0,"InternalQuery":"","Where":"","SchemaName":"","TableName":"","ColumnName":"","DataTypeName":"","ConstraintName":"","File":"drop_index.go","Line":522,"Routine":"dropIndexByName","UnknownFields":null}
+{"Severity":"NOTICE","SeverityUnlocalized":"NOTICE","Code":"00000","Message":"the data for dropped indexes is reclaimed asynchronously","Detail":"","Hint":"The reclamation delay can be customized in the zone configuration for the table.","Position":0,"InternalPosition":0,"InternalQuery":"","Where":"","SchemaName":"","TableName":"","ColumnName":"","DataTypeName":"","ConstraintName":"","File":"drop_index.go","Line":63,"Routine":"DropIndex","UnknownFields":null}
 {"Type":"CommandComplete","CommandTag":"DROP INDEX"}
 
 until noncrdb_only

pkg/sql/schemachanger/scbuild/internal/scbuildstmt/drop_index.go

@@ -59,38 +59,40 @@ func DropIndex(b BuildCtx, n *tree.DropIndex) {
 			pgnotice.Newf("CONCURRENTLY is not required as all indexes are dropped concurrently"))
 	}
 
+	var anyIndexesDropped bool
 	for _, index := range n.IndexList {
-		dropAnIndex(b, index, n.IfExists, n.DropBehavior)
-
+		anyIndexesDropped = dropAnIndex(b, index, n.IfExists, n.DropBehavior) || anyIndexesDropped
 		// Increment subwork ID so we know exactly which portion in
 		// a `DROP INDEX index1, index2, ...` statement is responsible
 		// for the creation of the targets.
 		b.IncrementSubWorkID()
 		b.IncrementSchemaChangeDropCounter("index")
 
 	}
-	b.EvalCtx().ClientNoticeSender.BufferClientNotice(
-		b,
-		errors.WithHint(
-			pgnotice.Newf("the data for dropped indexes is reclaimed asynchronously"),
-			"The reclamation delay can be customized in the zone configuration for the table.",
-		),
-	)
+	if anyIndexesDropped {
+		b.EvalCtx().ClientNoticeSender.BufferClientNotice(
+			b,
+			errors.WithHint(
+				pgnotice.Newf("the data for dropped indexes is reclaimed asynchronously"),
+				"The reclamation delay can be customized in the zone configuration for the table.",
+			),
+		)
+	}
 }
 
 // dropAnIndex resolves `index` and mark its constituent elements as ToAbsent
 // in the builder state enclosed by `b`.
 func dropAnIndex(
 	b BuildCtx, indexName *tree.TableIndexName, ifExists bool, dropBehavior tree.DropBehavior,
-) {
+) (indexDropped bool) {
 	toBeDroppedIndexElms := b.ResolveIndexByName(indexName, ResolveParams{
 		IsExistenceOptional: ifExists,
 		RequiredPrivilege:   privilege.CREATE,
 	})
 	if toBeDroppedIndexElms == nil {
 		// Attempt to resolve this index failed but `IF EXISTS` is set.
 		b.MarkNameAsNonExistent(&indexName.Table)
-		return
+		return false
 	}
 	// Panic if dropping primary index.
 	_, _, pie := scpb.FindPrimaryIndex(toBeDroppedIndexElms)
@@ -118,6 +120,7 @@ func dropAnIndex(
 		))
 	}
 	dropSecondaryIndex(b, indexName, dropBehavior, sie)
+	return true
 }
 
 // dropSecondaryIndex is a helper to drop a secondary index which may be used