MDEV-26450 fixup: Remove a bogus assertion

mtr_t::commit_shrink(): Do not assert that some previously clean pages
will be flagged as modified by this mini-transaction. It could be the
case that there had been no recent write-back of any of the undo
tablespace pages that we are modifying when truncating the tablespace.
It suffices to assert that some pages were modified again:
ut_ad(m_modifications).

This fixes up commit f5fddae

storage/innobase/mtr/mtr0mtr.cc

@@ -572,7 +572,6 @@ void mtr_t::commit_shrink(fil_space_t &space)
   ut_ad(!is_inside_ibuf());
   ut_ad(!high_level_read_only);
   ut_ad(m_modifications);
-  ut_ad(m_made_dirty);
   ut_ad(!recv_recovery_is_on());
   ut_ad(m_log_mode == MTR_LOG_ALL);
   ut_ad(UT_LIST_GET_LEN(space.chain) == 1);