cleanup: tests

sequence tests verify that one cannot change the structure
of the table. for that they need a valid alter table that
adds an index over an existing column. there's no column 'start'
in the table

mysql-test/suite/sql_sequence/replication.test

@@ -70,7 +70,7 @@ create sequence s2;
 alter table s2 add id int;
 
 --error ER_SEQUENCE_INVALID_TABLE_STRUCTURE
-alter table s2 add index ind_x(start);
+alter table s2 add index ind_x(start_value);
 drop sequence s2;
 
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