schemachanger: remove RBR fallback for add column

This patch removes the regional by row fallback for `ADD COLUMN`
in the declarative schema changer.

Epic: CRDB-31282
Fixes: #80545

Release note: None

Author: annrpom

pkg/ccl/schemachangerccl/ccl_generated_test.go

@@ -0,0 +1,23 @@
+setup
+CREATE DATABASE multiregion_db PRIMARY REGION "us-east1" REGIONS "us-east2", "us-east3" SURVIVE REGION FAILURE;
+CREATE TABLE multiregion_db.public.table_regional_by_row (
+  k INT PRIMARY KEY,
+  V STRING
+) LOCALITY REGIONAL BY ROW;
+----
+
+stage-exec phase=PostCommitPhase stage=:
+USE multiregion_db;
+INSERT INTO table_regional_by_row VALUES ($stageKey);
+INSERT INTO table_regional_by_row VALUES ($stageKey * -1);
+DELETE FROM table_regional_by_row WHERE k = $stageKey;
+----
+
+stage-exec phase=PostCommitPhase stage=:
+USE multiregion_db;
+SELECT crdb_internal.validate_multi_region_zone_configs()
+----
+
+test
+ALTER TABLE multiregion_db.public.table_regional_by_row ADD COLUMN w TEXT NOT NULL DEFAULT 's';
+----