txm: relax conditions for memtx_story delete

Since a transaction has been prepared its garbage (produced stories
and replaced tuples) cannot be deleted because they are recognized
as used in read-view even if there are no transactions in read-view.
This problem makes it difficult to test the memory monitoring system,
so this patch solves the problem.

Close #6635
Part of #6150

NO_DOC=no visible changes
NO_CHANGELOG=no visible changes