txn: erase old savepoint in case of name collision

Name duplicates are allowed for savepoints (both in our SQL
implementation and in ANSI specification). ANSI SQL states that previous
savepoint should be deleted. What is more, our doc confirms this fact
and says that "...it is released before the new savepoint is set."
Unfortunately, it's not true - currently old savepoint remains in the
list. For instance:

SAVEPOINT t;
SAVEPOINT t;
RELEASE SAVEPOINT t;
RELEASE SAVEPOINT t; -- no error is raised

Let's fix this and remove old savepoint from the list.