trx_purge_truncate_history(): Remove a debug assertion that
had originally been added in
commit 0de3be8cfd (MDEV-30671).
In trx_t::commit_empty() we do not have any efficient way to rewind
rseg.needs_purge to an accurate value that would satisfy this
debug assertion.
Note: No correctness property should be violated here. At the point
where the debug assertion was located, we had already established
that purge_sys.sees(rseg.needs_purge) holds, that is, it is safe
to remove everything from rseg.
736901b443