In commit 4c3ad244 (MDEV-27416)
an unnecessarily strict wait condition was introduced in the
function buf_flush_wait(). Most callers actually only care that
the pages have been flushed, not that a checkpoint has completed.

Only in the buf_flush_sync() call for log resizing, we might care
about the log checkpoint. But, in fact,
srv_prepare_to_delete_redo_log_file() is explicitly disabling
checkpoints. So, we can simply remove the unnecessary wait loop.

Thanks to Krunal Bauskar for reporting this performance regression
that we failed to repeat in our testing.