INSERT/DELETE/UPDATE followed by ALTER TABLE within LOCK TABLES
may cause table corruption on Windows.

That happens because ALTER TABLE writes outdated shared state
info into index file.

Fixed by removing obsolete workaround.

Affects MyISAM tables on Windows only.


myisam/mi_extra.c:
  On windows when mi_extra(HA_EXTRA_PREPARE_FOR_DELETE) is called,
  we release external lock and close index file. If we're in LOCK
  TABLES, MyISAM state info doesn't get updated until UNLOCK TABLES.
  
  That means when we release external lock and we're in LOCK TABLES,
  we may write outdated state info.
  
  As SQL layer closes all table instances, we do not need this
  workaround anymore.
mysql-test/r/alter_table.result:
  A test case for BUG#29957.
mysql-test/t/alter_table.test:
  A test case for BUG#29957.