Removed the flag that disables innodb on slave in the default
configuration of replication tests. That made the explicit
--innodb flag in -slave.opt files redundant, so lots of -slave.opt
files could be removed. Also, -master.opt files containing reduntant
--innodb flag were removed (those were redundant even without
changing the default). Removing .opt files is good because .opt
files cause server restarts and make tests less readable.
Also fixed a bug where rpl_innodb_mixed_ddl unintentionally
used myisam on slave.
