test(mysql-schema): fix sql query in userExists (#898)

Author: scrocquesel

sample-operators/mysql-schema/src/main/java/io/javaoperatorsdk/operator/sample/schema/SchemaService.java

@@ -76,7 +76,7 @@ public static void deleteSchemaAndRelatedUser(Connection connection, String sche
   private static boolean userExists(Connection connection, String username) {
     try (PreparedStatement ps =
         connection.prepareStatement(
-            "SELECT EXISTS(SELECT 1 FROM mysql.user WHERE user = ?)")) {
+            "SELECT 1 FROM mysql.user WHERE user = ?")) {
       ps.setString(1, username);
       try (ResultSet resultSet = ps.executeQuery()) {
         return resultSet.next();