This reverts commit e731a283.

A crash occurred during the test stress.ddl_innodb when
fil_delete_tablespace() for DROP TABLE was waiting in
fil_check_pending_operations() and a purge thread for handling
an earlier DROP INDEX was attempting to load the index root page
in btr_free_if_exists() and btr_free_root_check(). The function
buf_page_get_gen() would write out several times
"trying to read...being-dropped tablespace"
before giving up and committing suicide.

It turns out that during any page access in btr_free_if_exists(),
fil_space_t::set_stopping() could have been invoked by
fil_check_pending_operations(), as part of dropping the tablespace.
Preventing this race condition would require extensive changes
to the allocation code or some locking mechanism that would ensure
that we only set the flag if btr_free_if_exists() is not in progress.

Either way, that could be a too risky change in a GA release.
Because MDEV-24589 is not strictly necessary in the 10.5 release
series and it only is a requirement for MDEV-25180 in a later
major release, we will revert the change from 10.5.