Fixed main.innodb_mysql_sync failure.

TDC_RT_REMOVE_NOT_OWN_KEEP_SHARE is not same as
TDC_RT_REMOVE_NOT_OWN_AND_MARK_NOT_USABLE. Comment in sql_table.cc says:

      Storage engine has requested exclusive lock only for prepare phase
      and we are not under LOCK TABLES.
      Don't mark TABLE_SHARE as old in this case, as this won't allow opening
      of table by other threads during main phase of in-place ALTER TABLE.

At this moment we hold exclusive metadata lock, all we should do
is purge unused TABLE objects.

sql/sql_base.cc:
  Restore 5.6 behavior of TDC_RT_REMOVE_NOT_OWN_KEEP_SHARE.
sql/sql_base.h:
  Restore 5.6 behavior of TDC_RT_REMOVE_NOT_OWN_KEEP_SHARE.