MDEV-30100 fixup: Remove a failing debug assertion

trx_purge_truncate_history(): Remove a debug assertion that
had originally been added in
commit 0de3be8c (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.

storage/innobase/trx/trx0purge.cc

@@ -778,7 +778,9 @@ TRANSACTIONAL_TARGET void trx_purge_truncate_history()
         continue;
 
       ut_ad(!rseg.is_referenced());
-      ut_ad(!head.trx_no || rseg.needs_purge <= head.trx_no);
+      /* We may actually have rseg.needs_purge > head.trx_no here
+      if trx_t::commit_empty() had been executed in the past,
+      possibly before this server had been started up. */
 
       buf_block_t *rblock= trx_rseg_header_create(&space,
                                                   &rseg - trx_sys.rseg_array,