Removed redundant attempt to create mysql.plugin table:
- original code was supposed to INSTALL some plugins:
  INSERT INTO plugin VALUES ('innodb', 'ha_innodb.so'),
    ('federated', 'ha_federated.so'), ('blackhole', 'ha_blackhole.so'),
    ('archive', 'ha_archive.so');
- original code was supposed to fail if mysql.plugin exists:
  The query sequence is supposed to be aborted if the CREATE TABLE fails due
  to an already existent table in which case the admin might already have
  chosen to remove one or more plugins.
- mysql.plugin must've been created by preceeding mysql_install_db anyway