INT-4262: Fix SQL scripts for IF EXISTS clause

JIRA: https://jira.spring.io/browse/INT-4262

Since MySQL doesn't have `IF EXISTS` for `DROP INDEX`.
And on the other hand the entire `DROP TABLE` takes care about indexes as well,
we don't need the `DROP INDEX` command for mysql.properties
* Add `NODROPINDEX` variable to the `mysql.properties`
* Tweak `destroy.sql.vpp` to leverage `NODROPINDEX`

spring-integration-jdbc/src/test/java/org/springframework/integration/jdbc/DelayerHandlerRescheduleIntegrationTests.java

@@ -72,6 +72,7 @@ public class DelayerHandlerRescheduleIntegrationTests {
 	public static void init() {
 		dataSource = new EmbeddedDatabaseBuilder()
 				.setType(EmbeddedDatabaseType.H2)
+				.addScript("classpath:/org/springframework/integration/jdbc/schema-drop-h2.sql")
 				.addScript("classpath:/org/springframework/integration/jdbc/schema-h2.sql")
 				.build();
 	}