row_undo(): Remove the unnecessary acquisition and release of
dict_sys.latch. This was supposed to prevent the table from being
dropped while the undo log record is being rolled back. But, thanks to
trx_resurrect_table_locks() that was introduced in
mysql/mysql-server@935ba09d52c1908bde273ad1940b5ab919d9763d
and commit c291ddfd
as well as commit 1bd681c8
(MDEV-25506 part 3) tables will be protected against dropping
due to table locks.

This reverts commit 0049d5b5
(which had reverted a previous attempt of fixing this) and addresses
an earlier race condition with the following:

prepare_inplace_alter_table_dict(): If recovered transactions hold
locks on the table while we are executing online ADD INDEX, acquire
a table lock so that the rollback of those recovered transactions will
not interfere with the ADD INDEX.